The idea of a human being reborn into an animal form is something we've seen literally (with wargs) and metaphorically (Ramsay claims the women who give him "good sport" get "reborn" as his dogs after he murders them). But can we really say that Rhaego was sacrificed during, or for the purposes of, this ritual? I don't think we can. (For that matter, was Drogo's "life" still even a factor during this ritual? Not necessarily.) Right now, readers still have no idea what ultimately happened to Rhaego's body at all. We can't even confirm 100% that he's actually dead. But there's no mention whatsoever of his body being placed on that pyre (and given that Dany goes into great specifics about the objects on that pyre, you'd think her infant son's presence would have been noted). Not to mention, if Rhaego's spirit really was switched with Drogo's spirit in the tent (which, granted, wasn't necessarily the case) . . . then either the sacrifice of Rhaego could have played no further role on the pyre, as sacrificing and then re-sacrificing something seems like it defeats the point of a "sacrifice", or "Rhaego" was actually really sacrificed when Dany smothered Drogo's body, and Drogo's spirit played no role on the pyre.
Because Drogo's "spirit" had supposedly left his body at the time Dany smothered him. That was the whole point---whatever transfer Mirri effectuated, it wasn't enough to restore him to himself. Drogo was no longer Drogo when Dany smothered him. The spirit in him seems like it was either Rhaego's spirit (the spirit of a child) or the spirit of his horse, but either way, the flesh on that pyre does not appear to have necessarily held Drogo's actual essence as its last "occupant" before death.
So there are explicitly three "bodies" on that pyre: Drogo, Mirri, and a dead horse. (Note that this horse is not Drogo's stallion, but rather, a horse from the small herd that remained after the khalasar broke up---this is one of the horses the new khals didn't take, so presumably it's one of the "lesser" horses). I think no matter which way you look at it, three human lives can't be said to have been sacrificed for that pyre. At best, you come up with two humans and a horse. At worst, you come up with two horse "souls" (the horse on the pyre, the horse in the tent that might have gone into Drogo's flesh) and a human being (Mirri).
(As a side note, if Drogon has the spirit of a horse, it might actually explain a number of things about him. Why he lets Dany ride him without any magical implements. Why he responds to a whip in the pit when we know the whip doesn't actually hurt him. Why he camps out on the Dothraki Sea instead of attacking villages and neighboring cities in Slaver's Bay (which Drogo's spirit would logically have inspired a dragon to do)---it's not the dragon's spirit, it's not Drogo, it's all about the horse. And actually, given the bits mentioned upthread about a connection between the fall of Valyria and the rise of the Dothraki . . . perhaps ancient Valyria originally "tamed" dragons by effecting some type of horse-transfer, a transfer between an animal that was ordinarily ridden and this wild fire-breathing monster? No evidence there, just thinking aloud.)
I think this discussion would be enhanced by an analysis of what was "actually" going on in Drogo's tent, as that event served as our only precedent for Mirri's magic. (Not to mention, look at the physical parallels: the tent involved Mirri, Drogo, and a horse all inside the tent, surrounded by shadows, with Dany and the khalasar outside; Jorah then carried Dany into the tent. The pyre involved Mirri, Drogo, and a horse together on a platform surrounded by flames, with Dany and the (lessened) khalasar outside; Dany then walked into the flames under her own power.).
Mirri claims she wanted all along to destroy the Stallion that Mounts the World, that Rhaego was always going to be the "death" needed to save Drogo. She says that a mere horse would never have been enough. It's assumed that, by carrying Dany into the pyre, Jorah inadvertently condemned Rhaego to death, as being in the tent is assumed to be necessary for the spell to work. But that doesn't necessarily make sense, because Mirri is the one who actively prevented Dany from remaining in the tent during the ritual:
"I will stay," Dany said. "The man took me under the stars and gave life to the child inside me. I will not leave him."
"You must. Once I begin to sing, no one must enter this tent. My song will wake powers old and dark."
If Mirri's spell relied all along on Dany's (and therefore Rhaego's) presence in the tent to effectuate a Rhaego/Drogo transfer . . . why on earth would she take such steps to prevent Dany from being in that tent during the spell? It seems incredibly counterintuitive.
I think there are two options here: 1) Bringing Dany into the tent didn't cause Rhaego's death, because Mirri was perfectly capable of using him as a sacrifice even when he was outside the tent's confines, or 2) The horse actually was meant as the sacrifice all along, and Rhaego either isn't dead or his death played no part in this ritual. Both options have interesting implications for what happened on the pyre.
Option 1) implies that Mirri's magic has a precedent for affecting someone physically outside the confines of the place where magic is being performed. In both instances Dany entered, against all counsel, the physical place where the magic was being performed---the physical confines of the tent, and later the flames of the pyre. But if Mirri's magic really could affect Rhaego inside Dany's body when both were outside the tent, couldn't her magic logically have also been affecting Dany when she was still outside the pyre? And look at the magic actually used in the tent: a human life initially found outside the tent (Rhaego, who was in Dany's womb) was supposedly exchanged in order to preserve a human life found inside the tent (Drogo). It seemingly wasn't intended to birth Rhaego (at least, not in a way that would lead to him surviving) or to resurrect Drogo (as Drogo wasn't dead yet).
But if Mirri's magic was able to affect Rhaego outside the tent all along, why did she bother telling people to stay outside? If it didn't matter if Rhaego was inside or outside the tent, as Mirri was able to kill him either way, (as events imply to be a possibility), why bother kicking everyone out of the tent in the first place? Perhaps the reason was simply that Mirri didn't want any interruptions. If Dany had been inside the tent at the start, she might have done something to interfere with Mirri's song, especially once the pains started, and Mirri could have wanted to avoid that. If that's true, then the fire's effects on Mirri's song---causing her to switch from singing to screams---could be an issue.
Option 2) can answer a number of issues regarding Mirri's ultimate "plan". Clearly she never intended on Drogo returning "as he was". It's not in her interests or the interests of her people to have Khal Drogo rampaging again. If she really thought the ritual wouldn't work with a horse, and she needed Rhaego . . . why kick Dany out and specifically tell everyone to stay outside, leaving her alone with just the horse? And if someone's presence in the tent mattered (and there's a chance that it might have) then given that Mirri forbade every human being from entering the tent, perhaps the whole plan all along was to switch Drogo's life for his horse's life, so that Drogo ended up with a "lesser", inhuman life-force. But if her "targeting" was foolproof, why would it matter if there were humans in the tent? Perhaps Mirri was afraid that her magic might seek a different target if other people were in the tent along with the horse, and that's why she kicked everyone out. If Jorah had dropped dead in that tent, or even Dany herself . . . perhaps Khal Drogo actually would have been restored to full health (an adult human life exchanged for an adult human life, especially in the case of Jorah, who was a warrior and (at one point in his past) a leader of men, like Drogo). And Mirri of course didn't want that.
And I think the issue of what really happened to Rhaego (is he alive or dead?) would logically inform Mirri's choices vis a vis the pyre. If Rhaego is alive---if someone grabbed him and took off when the khalasar split, and Jorah didn't tell Dany because he knew she'd have had no real way of either finding or rescuing him---then in MIrri's eyes, the Stallion would still be a threat. How to counteract him? Well, dragons would do it. But dragons in the control of Rhaego's mother . . . that makes no sense. Dragons in the control of Mirri Maz Duur, in the control of the Lamb Men, would make more sense from Mirri's perspective. (Remember what the Valyrians were before they got dragons: shepherds.) But if Rhaego is dead, introducing dragons into the equation makes far less sense, from Mirri's perspective.
We know Mirri knows Marwyn, a Westerosi maester. It's very possible she has knowledge of Westeros and the Targaryens. What were the Targaryens known for? Not for hatching dragon eggs---quite the opposite, actually. They were known for getting themselves killed in rather spectacular ways while trying to hatch dragon eggs. If Mirri wanted dragons for herself, first she'd need some eggs. Dany's eggs weren't readily accessible. A scenario whereby Mirri manipulates Dany into getting the requisite elements together, with Mirri expecting that Dany will get herself killed in the hatching process (as Targaryens are wont to do), thus providing a necessary blood sacrifice and leaving Mirri Maz Duur with some brand-new dragons, sounds like a more reasonable interpretation to me than a scenario where Mirri works hard to give Dany dragons. Because even if you disagree that Mirri wants Dany to suffer . . . she keeps, rather rightly, calling Dany a naive child. And working hard to give a naive child fire-breathing death machines strikes me as absurd.