Tom, welcome to the forum! We bought our 2004 LY 30 ft w/slide (gas) about two years ago. We found that this site seemed to give reliable prices: Prices & Used Values for RVs, Travel Trailers & more - NADAguides
One caution, the site shows a lot of equipment as optional that is standard equipment on the coach. You don't want to "double dip". Establishing a price from this site is easier if you can get a copy of the window sticker on the unit you are considering. Also, this site is a good reference: Airstream, Inc :: Service
Good luck with your search.