I like the idea a lot... Sometimes I don't have a personal project in mind and I'd like something more guided with which I could practice.

However, as others have said I'd need to see what I'm getting for the price. Particularly I'm interested in how the spec is laid out, and how the solution is laid out... From university I learned there's an art to building a pset where you give enough pointers but do not hold the student's hand's too much and that's what I'd be looking for - the right level of challenge, in a nutshell.

Maybe you should give out a project for free, or create a sample or similar...