I suspect the Lannisters are broke

[LucionLannister]

I think there's a long league's worth of difference between being broke and being niggardly. Tywin not wanting to pay for a wedding (a wedding that technically had no actual Lannisters getting married) doesn't mean he wasn't able to. Not wanting to mix the treasuries of the Iron Throne and Casterly Rock doesn't mean Casterly Rock doesn't have any money.

And hiding the fact that the gold mines have all run out wouldn't be that easy. Casterly Rock is a HUGE gold mine. Which means it probably employs A LOT of people. People to mine the gold, people to move the gold, people to maintain the mines, etc. If Casterly Rock suddenly stopped hiring people to do this, people would know.

And like it was stated, Kevan seems assured he could pay the whole of the Iron Bank's debt out of pocket, and didn't seem over concerned about the long term finances of House Lannister. Tyrion also promises huge sums of gold to sellswords if they help him win Casterly Rock, and while Tywin probably didn't have money chats with him, he did LIVE in Casterly Rock prior to the start of the series. Tyrion would be smart enough to know if the mines in his own castle weren't working.

[Siddard Stark]

i think people are getting influenced by the show where Tywin stated that House Lannister is Broke. It is clearly stated in Kevan Lannisters POV that House Lannister is pretty stable he just doesn't want to lend more Lannister Gold to the crown as he knows hoe pathetic the financial situation of the crown is. Both Kevan and Tywin know that the war has to be completely over an d all rebellions should be put down then and only then can the crown make some money. Keeping standing armies is quite expense for the crown. And further winter has come to westeros and with the recent war there will be a massive drop in resources which will cost the crown a lot and they would have to buy food to support the smallfolk during the winter!!!