Interesting idea. I think it's a little too generous as proposed. I would do it as each $50 enters you into a drawing for a free track day. And then have a certain number of winners based on how much money is raised, e.g. 1 winner for every twenty entries. Or maybe a donors' track day where all donors above some minimum level ($100?) can come ride for free. Would something like that get people a little more excited?
Other ideas?
Dirk