Jamie is an excellent treasurer because he is good with money and a cheapskate.
A treasurer is a person responsible for keeping track of money.
When Claudia decides to run away because she is underappreciated in her family, she chooses her younger brother Jamie to accompany her. She knows he is the right brother because he has money. She chooses him because he doesn’t complain, he is funny, and he is rich since he “saved almost every penny he got” and never wasted it on things like baseball cards. As a result of his gambling, he has “Twenty-four dollars and forty-three cents.”
Claudia appointed him treasurer; he would not only hold all the money, he would keep track of it and pass judgment on all expenditures. (Ch. 2)
Jamie is good at keeping track of their money, lamenting at how much things cost. Even when they find the money in the fountain, he still worries about how much they have.
Although Jamie seems like a sweet little boy, he is actually quite a hustler. He gambles with his friends and apparently wins more often than he loses. He almost never spends the money he earns.