[Tendrak]

I've hosted a few reward contests like this. My advice is to pool more prizes with other people willing to chip in for a good cause like this.

Also make a second and third place. This will encourage more participation.
Even if first is still $50, second is $10 and third is a buy one get one free pretzel at the mall.

If you can get the first prize higher more people will make time to take a shot though, and you'll catch bigger fish too. This is just in my experience.