I agree, reward for quality, not quantity.
If you are interested in community building in addition to increasing post count, you might wish to consider a more commmunity oriented incentive - with money it's easy to take your prize and move on, and monetary incentives don't always attract the best type of posters.
Have you considered doing a user spotlight or something similar where the incentive is intra-community recognition? An incentive that requires caring about the community to be appealing?
I guess I'd worry about a money prize as well, because no matter how clearly you lay down rules there will be people who just see, "Win $50," and spam the hell out of your forum without understanding the requirements to win.
The forum I moderate provides incentives for participation in the way of forum currency that can be exchanged for avatar enhancements, and despite the fact that spamming in the wrong place will loose your account and all the currency you have accumulated, spam is still a rampant problem - people's greed overwhelms their common sense.