I'll start by saying that I really despise encouraging participation in things by awarding prizes, especially money. That to me is over the top, especially for things that are supposed to be community driven like this site. I was poking around several different meta's on the sites where I typically lurk and some of the smaller ones ran Topic of the Week contests to drive up participation and/or expand the scope of the site by encouraging questions on less-frequently used tags (UX, SciFi, Cooking). The one on Judiasm works without any prizes at all.
I certainly am not advocating prize money, I think that's a terrible idea. And this site doesn't need it really. But would it be something worth doing/entertaining to have a Topic of the [time period] type of recurring event?
The closest I would say to a prize would be recognizing the questions and answers under the topic that got the highest votes during the time period -- the high voting gives rep points which is essentially the only reward system in place on the site. Or if individual sites can award unique badges, then a badge could be given for best answer/question under a specific topic contest.