Choosing the right prize is a key element to running a successful contest. It's important to choose a prize the makes your contest attractive to designers, remember that you are competing for designers attention!
Because the nature of design work varies so much from category to category, the answer to "how much is the right prize amount?" is different for each category.
¹ Prizes in the Web Design category are for the design of a one page site, this excludes coding such as HTML and CSS.