I spent a lot of money trying to get a cheap resistive touchscreen, because (at the time) the commercial ("real") ones were on the order of $900 for a 17" screen.
This might be too small for what you're trying to do, but HP is manufacturing a 21.5" lcd monitor with multi-touch technology built in for about $300. I've installed about 10 of these- half of them in a very harsh manufacturing environment where I didn't expect them to hold up, and they are performing surprisingly well. The model to look for is L2105tm.
Under XP there are HP drivers that will get you single touch. Under Windows 7 you get two points of touch. (I forget what vista support was). The monitors are designed to report up to four points, but I don't know how you get that to work. Dual-touch-screen works with this model under windows 7 (even with rotation) but XP will get confused if you connect two L2105tm's to one computer.
Hope that helps someone.
Jeremy