Money, money, money. For a book about kids who make mere cents per week (literally) in allowance, there's sure a lot of mention of cash going around in From the Mixed-Up Files of Mrs. Basil E. Frankweiler. From Jamie nitpicking over their every purchase when they run away, to the angel statue's bidding price of $225, to Mrs. Frankweiler's obviously rich home, money is always a part of the conversation. The idea of what "rich" is also comes up a lot; Jamie is considered rich by Claudia because he's saved up over $24 in allowance money. But Mrs. Frankweiler is on a whole other plane of wealth, and the kids get to take a peek into that world as well. It's all relative (pun intended).
Claudia runs away because of the injustice of her financial means, but in the end, she finds that material wealth is less important than acquiring new secrets and experiences.
Mrs. Frankweiler may be rich in financial terms, but she's lacking in a lot of things that qualify as real wealth.