IMHO - one with working appliances, probably needing tires, possibly axles and brakes, body repair required, I'd not want to pay more than the low two thousands at most. I've seen several over the past year on Craig's list in what would appear to be better physical condition, in the Pacific North West, asking in the 2 to 3K range. However, I've not gone to view them. Some were for sale for a LONG time.
Being very thrifty and doing all of the work myself I got mine up and rolling for about 2.5K after initial purchase. Mine already had a brand new axle, brakes, shocks, tires and rims, propane bottles and regulator and hoses and everything but the water heater functioned after some work to get them into shape.
I didn't check the water heater but accepted the fact that everything worked
. The refer and stove worked fine, the furnace needed some attention before we could use it safely. That's fine and expected. But the water heater.
It had propane running to it (discovered later that it was plugged with a brass fitting), it had the housing nicely installed and the vent door looks good on it. However, the guts of it were AWOL completely. Sigh, the shell was empty. There was no hot water heater left. I suspect the tank burst and they removed the guts and it was being used to store an air mattress. When the PO's said everything worked they were being a bit enthusiastic.
I'm still happy with what I ended up with, and I have just over 7K in the trailer now. I'd of never paid what I paid though if it needed the axles, wheels, brakes, or body work. I would have deducted the cost of all the components it needed from the asking price and made my offer and walked if they didn't take it. There are a lot of Airstreams for sale and it takes patience and being willing to negotiate to get the one that works for you.