This is a great idea, no legal issues. The important thing is that since the candidate loses 2 hours of his time working on the problem, the employer has to lose something equivalent of that too.

I would be much more willing to do the project if I knew the employer would lose $200. Among other things, this shows that I'm valued.

That's a great idea! Not only does it just 'feel right', it also prevents some of the legal mess people were seeing with effectively moonlighting for the hiring company.

That is a really good idea.