Buy where you see value. Most of the machines I think are under priced tend to be in the sub-$1000 range or $2500 to $3000 range. If I were buying now, I'd be looking at something from the early 80s or World Cup Soccer. I also think I own some of the better bang for your buck games.

This might be too obvious, but in a lot of areas you kind of have to just start with a budget and see what turns up in your area. Set a budget and look for a machine that's price fairly.