- Analyze what calculations your users are doing and serve related ads. Example, for someone doing compounding interest 12 times, you show books related to investments. Or someone using some mathematician work, a link to his biography book;

- Contact a company and ask if they want to sponsor a contest. The first 5 users to complete some math problems will earn whatever the company is offering (essentially that's an advertising campaign for the company);

- Run a contest between your users. They pay 5 bucks to enter, and the winner gets the money (after your share ;). A tournament perhaps?;