With regards to the prize; usually when people submit a design they would include maybe a text logo that fits in with the design they have made that would be the $100 option. If you want a "real" (can't think of a better way to describe it) logo included, ie vector with symbol and some real thought put into then expect to pay $150 ($100 for the design, $50 for the logo).
At the end of the day this is your contest, the prize is yours to decide just remember though that you get what you pay for