Might be above your cost range, but we've been very happy with the Outback track systems. Effectively it's a Brecoflex self centering belt riding on pulleys between milled aluminum, but it could probably be simplified down to a brecoflex belt on pulleys with some cheaper structure. If you're interested, Outback MFG is a Brecoflex reseller, so you can get some FIRST legal belts through them. (You'll need to contact them for details... I'm not current on what belts they carry in stock, or costs, or even availability...)

If we had more information about your application, I'm sure we could help you out with more accurate information....