It is true that you have to kiss a lot of frogs when you're looking for a Classic Airstream Motorhome. The fact is, there weren't that many made (as compared to trailers) and with all of the additional systems included in a motorhome of this age they all tend to need work.
A few "general rules":
1. Shorter motorhomes go for more money as fewer are available and they tend to be more popular than the longer floorplans (specifically 325s and 345s which both have tag axles).
2. Rear bedroom units in shorter floorplans (310s) tend to be the most desirable combination of length and layout and therefore the most expensive.
3. Larger units like the 345 are more "available" and therefore less expensive.
4. Later model units will likely have less wear and tear than older units
5. Scratches, dents, repairs, mechanical issue are to be expected in any motorhome of this age. You need to plan for this in your inspection and in your budget.
6. Honestly, 21K for a quality motorhome like a Classic is not a lot of money when you compare it to the price of a cheapo mid 90s Class C with absolutely no "curb appeal"
7. Mechanicals and systems can be (relatively) easily repaired or replaced but body damage, especially in the corners and caps, can be very expensive to repair. Looks for good shells and complete working units...then work up from there.
8. Prices tend to be coming down given the glut of motorhome sales and rising gas prices. I expect the recent reduction in pump prices to drive up confidence and moho purchase again so keep that in mind.
9. Dealers are often more negotiable than private sellers as they have a more "business (read realistic) perspective" about unit value. Many times they will take a trade on a Classic for very little actual cash to get an upgrade to a six figure coach. They also don't like old inventory so prices can be very negotiable. Private sellers often know the market value is beyond the book value (from sites like this one) and can ask "demand value" for their coach - especially nice ones.
10. There are deals out there to be had. In the last year I've seen a 96 360 go for 15K a 79 Argosy go for $1500 and a very
nice 280 go for under 20K.
11. (bonus)....like the old real estate saying, but a little different; Condition, Condition, Condition. While many systems and mechanicals are available and repairable, especially if you're handy, the overall condition and maintenance history of the coach will utlimately dictate the price. Compare this to what you really want. If price is an issue but sweat equity is a possibility then don't be afraid of the fixer uppers, but if all you want to do is travel and don't know which end of the screwdriver is up then pay more for a complete unit, with solid maintenance and newer (already replaced) systems. I've seen workable 345s go for $10K and perfect ones go for $45K. Condition is everything.
Basically, you need to keep looking. I looked at many units before I found "the one". Granted I was seeking a basket case in a good shell. But, after looking at more than a few (one as far a 1000 miles from my home) I knew the right one as soon as I saw it. Make sure you qualify sellers with lots and lots of pictures and as much disclosure as possible. Skins can be repaired...but I agree than a unit with major body damage and a bad engine is not something that should be in the 20K plus range.
Good luck with your search. Make sure you hit the classifieds section on this site as well as www.airstreammotorhomes.com
in addition to eBay as resources for units up for sale.