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.