Sorry but I think the 15 fee is a bit much... Interdressage charges £10 per test, but it seems better run IMO as they explain everything well in a detailed way (regarding rules, tack etc.)... Who are you planning on having as a dressage judge?
If I'm right this is just a preview of a "draft" of the final site - is that correct? In which case I'm looking it as a site that isn't ready to go live yet, and has some more info to be added. In which case I think it's very promising.
I think you might need known names as judges, or people might not take part.
20% of the entrance fee sounds very little for a prize fund. I mean, what are the overheads? They don't have to pay for facilities, judge travel (judges are usually only paid their travel expenses and maybe a bottle of win/flowers if they are lucky), ambulances, refreshment stands. The only cost is that of the webspace (maybe £5 per month, if not free) and pay someone to judge (which they can do from anywhere during their free time).
It hardly seems value for money to me; it just looks like a money-making scheme.