I’m going to put some Math Studies SL project ideas. If you have some more, let me know on the commentary page or by e-mail.
Derive a mathematical model for prices on internet auction services. For example, you might want to investigate the relationship between the last bid and time left to the end of the auction. Another idea in that direction is to choose some particular good which has some quantitative feature (say, mp3 players with the amount of memory) and discuss the correlation between price and that feature, possibly finding some best-fit line.