Its my observation that airstreams depreciate at least 12-15%/year from list price of the most recent model year. For instance if a 2010 25FB flying cloud is selling for $62,400 to be priced competitively a 2008 should be in the low $40,000s. An ideal price would be in the high $30s. From there you need to consider color (personal preference), condition and what options are on board before making your offer.
As they get older the depreciation rate compared to new list price will slow down to the 8-10%/year range. You will see trailers in the 10 year and older range developing a floor around $15,000 or so but this will of course vary with length. My research hasn't gone beyond 15 year old units.
You can use this as a guide if you wish, but as they age past the 5-6 year mark the comparison to new loses its relevance and it becomes more and more important to comparison shop. $/foot is a good comparison when comparing same models. Just remember the shorter the unit the higher the cost/foot. But, length can change the way things work. Most of my research is on the 25 footer. I think its the best offering AS has with regard to space, weight and tank size.
Valuing a trailer is complicated and I am offering "rules of thumb" that I am developing for myself. There are several good deals advertised on this site right now, but there are far more offerings that need to be negotiated down.
You dont have to get the best deal in the world, just make sure you are happy with what you paid and you can handle the deal over time. A new one seems to be out of reach for me but I plan on buying close to new in the next year or so.
Right now my problem is my 93 aluma lite. Its a great rig and we are enjoying it immensely! Book on that is $2500 or so. An AS will hold value better than that.