Well, price all depends... Namco arcades has some great deals from time to time. They sell complete, working machines at great prices. Unfortunately, the Minnesota selection is a bit wanting (unless you want Lucky and Wild) but play around with it and check back. Here is the URL:
http://www.namcoarcade.com/gamesearch.aspIf you want a barebones, no monitor, just plain ol' cabinet, I wouldn't spend more than 50$. This is a great way to go if you want to put in a PC monitor instead of an arcade monitor.
Think of the arcade game purchase like you would a car purchase. Is it in good shape, does it work, and does it have everything I want or need. and price accordingly. Don't jump on the first thing you see, it's often the worst deal. And if you have any more questions, this is the place to ask.

Good luck!
APf