She wasn't, she always knew he was extremely rich, even when she turned down his first proposal, that was a joke. But her view of him did change after the letter, and changed even more after seeing Pemberley, not because it showed proof of his wealth, she already knew that, but because she saw how he treated his tenants, servants, land property, what his taste re landscape, decor etc was and so on. If she was mercenary she would have accepted Darcy's first proposal (where she met his rich titled relatives and saw their great estate: Rosings which was likely roughly equivalent in value to Pemberley, and there's a sharp contrast between the taste of Pemberley and Rosings) or she would have even accepted Mr Collins proposal where she would have been able to keep Longbourn after her father died and then she would have stayed rich at the level of her father, but not the level of Darcy.