I'd agree $500 is a very steep for the Cresta. However, it is a dedicated full tourer, whereas the Trek isn't: 1 set of water bottle bosses, sidepulls, lack of braze-ons, higher gearing (I'm guessing).
Consider what you paid for the Trek + the $100 the LBS is asking for the Cresta. You'd be getting a full touring bike in your size for that price. Is it a good deal? In addition, if the LBS performed a FULL overhaul, it's well worth $100, as any used touring bike will require the same.
Your other option (aside from a trade) is to negotiate an even trade with the LBS or sell your Trek on craigslist or eBay and look for something else. Vintage Treks have a bit of a following, but I don't think your model would sell for much more than a couple hundred dollars. The vintage Trek touring models that sell best are usually the 720s and 520s, and the 'race' models with full Reynolds 531 or Columbus tubing.