On another site the contests seem to be about $100 in store credits and some content for 1,2,3 and 2 honorable mentions. Renderosity contests seem to be a bit better in prizes. I doubt myself if the prizes really effect the entries since I never expect to win I do it purely for the experiance. 

I think $50 would be a decent prize pool $25, $15, $10 for 1,2,3rd.

If you want to run it at renderosity contact an admin.