In the last year here (Austin), I've hosted a half dozen hackathons with no prizes.. not even silly prizes. It takes minimal effort getting 40-50 people, and some effort to get 100. On the other side, our last major social good hackathon (Hack for Change) had nearly 250 with no prizes announced up front.

(We did award SXSW 2015 badges to the "audience favorite" but that was a surprise at the tail end of the demo time.)