Besides, he was rich; unlike most boys his age, he had never even begun collecting baseball cards. (1.2)
Jamie is the first character in the book described as rich—and it's because he has a lot of allowance money saved up. That might not count for much in the adult world, but it sure does to Claudia, who can't even save up for a sundae sometimes.
"Well, you see, Claude," Jamie whispered, "I have quite a lot of money."
Claudia thought that old Jamie would end up being a business tycoon someday. Or at least a tax attorney like their grandfather. (1.44-45)
In the book, it's obvious that some people have a better awareness of money, and others don't. Even though Claudia is older, she wisely realizes that Jamie is the one who has a knack for making and keeping track of money. And she… well she would probably blow it all on ice cream.
Claudia appointed him treasurer; he would not only hold all the money, he would also keep track of it and pass judgment on all expenditures. (1.60)
He who holds the money also holds the power—or something like that. Claudia may be the bossy older sister who plans the whole trip, but she lets Jamie have a say, too. He gets to stop her from making stupid purchases.
"That's right. You might, but most people wouldn't. Well, if this statue is by Michelangelo, it's worth about $2,250,000 instead of $225. That's the same as saying that suddenly two and a quarter cents is $225." (4.49)
Geez. Talk about getting some bang for your buck. If the museum really did get the statue for $225 when it was worth millions, then they'd be getting the best deal since Groupon came along.
"Someone very rich must have tossed in this quarter," Jamie whispered.
"Someone very poor," Claudia corrected. "Rich people have only penny wishes." (5.96-97)
It's interesting to think of what it means to be rich. Does it equal happiness or fulfillment? Here, Claudia seems to think that rich people don't have that much to work for, so they toss smaller coins in the fountain. Poor people toss in bigger coins. It's a high stakes kind of wish for the poor.
Paying four dollars and fifty cents for empty space had been hard on Jamie. Claudia knew they wouldn't take a bus back to the museum. They didn't. (7.25)
It's kind of a bummer when you think of how much money we have to pay for empty space. P.O. boxes, rent… seriously, if we can't eat or play with it, how is it worth anything?
Jamie sighed, "I gave him seventeen cents. So it wasn't such a great tip. […] We're broke. How do you feel about that, Miss Taxi Rider?"
"Pretty uncomfortable," she murmured. "There's something nice and safe about having money." (9.12)
One of the adult lessons that the kids learn is that it's nice to have money, and kind of uncomfortable and scary to not have it. When she realizes that all their money is gone, Claudia loses some of that puffed out chest.
Even in this very elegant house of mine, that bathroom is especially grand. […] There was nothing she wanted more than to take a bath in that tub. (9.79-80)
Mrs. Frankweiler's house is not only the lair of secrets, but it's also the lap of luxury. For a girl like Claudia, these fancy baths and grand dining rooms are something she could get used to.
"I'm not going to give you the sketch outright. I'm going to leave it to you in my will. You won't tell my secret because if you do, I'll write you out of my will. You will lose all that money." (9.231)
Holding money over someone's head as bribery—what an age-old tradition. Mrs. Frankweiler decides that she's going to leave a bunch of money in her will (in the form of the sketch) to Claudia and Jamie, but that they can't tell her secret.
"She didn't sell it for the money. She would have shown her evidence if she really wanted a big price. She sold it for the fun of it. For excitement." (10.13)
At the end of the day though, money isn't everything. In fact, for Mrs. Frankweiler, she can toss around the money however she wants—she's practically bathing in the green. She might as well auction off the statue for a loss so that she can laugh at all the idiots who didn't know how much it was worth.