XT 550?!?! You really mean the XT 550 that was last made in, what, 1982? Sure, any modern designed bike will be better than an XT550.
There are no NADA guidelines; it is a free market, with prices based on availability, location, season, etc. For real world pricing, search the classifieds at Advrider.
If you're not in a hurry and you live in a region affected by harsh winter weather, you might as well wait for a couple more months when the prices typically fall.