I guess the selection that will occur based on the price can be either positive or negative depending on your taste.
One of the reason to join one of these communities, I would think, is to socialize and network with interesting individual... and you might want to pay extra for that. Then again, if the prize de facto excludes the people you find interesting than not so much :)
I think I'd try to do a little prize segmentation with tiered costs, that way you could perhaps get more interesting people and maximize profit. Then again maybe it will book fully using these prizes