FWIW, the prize/incentive model for "Wiki takes X" has been done before:

http://en.wikipedia.org/wiki/Wikipedia:Wikipedia_Takes_Montreal

But if you are really going the prize/incentive route, you need to be prepared to answer a lot of questions, and provide results to justify involving money . As a coordinator of the the ongoing TamilWiki Media Contest, i have been facing similar questions. At the end of the day, the event needs to produce tangible results for wikimedia projects in form of usable files uploaded to commons.