Heya,
in the past couple of years, I picked up a 6x7, 6x9 (23C), 4x5 and 5x7 enlargers. Total price: $40.
I had to upgrade some lenses ($25 for a Nikkor 50mm 2.8 and $20 for a schnieder 105mm). Add some lens boards, and easel and other stuff and the price starts to come up a bit--but, the basic lesson is that there are more enlargers in the world than people who want them. If you spend real money, get good quality (I have to remove the dead spiders from mine...).
Things to look for
1) good lenses. Don't get too stressed on this, as you can buy them off of eBay for reasonable (read--ridiculously low) prices. The Beslar lenses are not highly rated.
2) look for a setup with a 105mm lens, if you are going to do 6x9 work. As noted above, you can get them reasonably on eBay.
Ask when the owner last used it. Ask if it works. If it is a condensor-type, there isn't a lot to go wrong. I have heard that the 23C can have bad gears for raising and lowering the head--people lock the gears then make them move anyway, stripping the plastic gears.
The photo-fair is a good suggestion. The prices are not rock-bottom low, but they are good. Be careful, though, when I was first setting up my 23C, I went to the photo-fair. I was so eager to do something *NOW*, that I got a bunch of stuff, including a Nikkor lens. Paid too much at $40. Paid way too much at $40 considering that it was the 50mm f4 model. Not a bad lens, but people can't give them away on eBay.
Try to get as much stuff with it as possible---lens carriers, lens, lens-board, filters, trays, etc. You can find the stuff easily enough (unless you get a schnieder 105mm lens---which takes a small jam-nut and lens-board), but when you get the enlarger you will want to make prints NOW!
Matt