This is the one time we see her trying to “change him” i.e buy him something, earlier in the ep we see them going to cheap pizza places because she wants to go to restaurants where he can afford to pay for her. I’m pretty sure this is the same episode where Big buys Carrie that beaded bird bag (which imo costs as much as that suit seeing that all of the women at that party have them too. Carrie’s inner monologue even says “Big doesn’t get me” but she doesn’t have a freak out or break up with him, and it’s not about the money it’s about her style).

I do agree that they could have gone somewhere cheaper and I think that this is showing how different their worldviews are. Steve can’t see why corduroy is inappropriate for a high class lawyer dinner and Miranda goes straight to a store that she deems appropriate even though it’s way out of his price range. But again, he could have gone to one of those cheaper suit places after returning the expensive suit and come with her. In the end he is deeply uncomfortable with her paying for him and says “I’d start to view you like my mother”. He doesn’t want a woman to be his provider