Alex is also a little fast with the "Most" part there. Around here, it's more typical to charge $10 for a contest, and they have been all sanctioned.

Food isn't included though, and different clubs in this area deal with that differently or not at all.

So it's really up to you. Your basic costs are going to basically be the santion fee plus streamers. You can get pre-rolled streamers from Hattrick at about 50cents per streamer (including shipping if you order a good sized pile), and you want to have one streamer per pilot per round. So for a typical contest around here, it's usually 4-6 rounds per class (4 or 5 rounds if there are 2 classes being run, 6 if you're just doing SSC usually, but you could do more if you want). Anyway, 6 rounds is $3 in streamer, plus a little money for string, though that's generally a dollar for a spool that will last a while.

I've been running the contests here basically at a break-even level, which has been working for us. If you want to charge more, you can, it's up to you.