Touchscreens are not really cheap to buy no matter where you buy them from.
About the only way to save a few bucks, is to buy a overlay touchscreen adapter and put it on a monitor you already have. And trying to find a monitor with a S-video input can be a problem, although there are adapters for that also.
And even doing it that way, usually somewhere costs around $200 just for the touchscreen overlay panel.http://www.magictouch.com/addon.html
Plus any needed S-video adapter will also add to the cost as well.'
The other option is fleabay sellers, but be aware, many of them are not selling genuine Escient touchscreens. And that some are selling setups that also require the S-video adapters, however most will sell you the needed S-video adapter for about $50 extra over and above the price of the touchscreen setup they are selling.