Something smells fishy... There is a belt tensioner that should keep the belt tight and maybe this is a problem. (?) Timing belts just don't stretch a whole lot so a new belt isn't going to be much "tighter". I'm not that familiar with the Zetec, but $500 sounds way out line, especially since the belt looks to be completely external. From your pics it doesn't look that bad, but you have to look at every inch carefully for cracks or "chunking" to know for sure. Might try a post in the Zetec forum and see how many DIY'ers have done it and how long it takes. I doubt the belt/part would cost more than $120 or so.
