E.L. Konigsburg’s From the Mixed-Up Files of Mrs. Basil E. Frankweiler
From the paper:
Oscar Wilde asked, and then answered his own question: A man who knows the price of everything and the value of nothing. This book is all about the difference between money, on the one hand, and value. Claudia takes Jamie with her because of all the people she knows he has the most money. And they become fascinated with solving the mystery of the angel because they think it will lead them to great riches.