Look at similar models for sale on the Airforums Classifieds, RV Trader, RVT, Etc. This should give you a ballpark figure.
If it has been sitting since 2011 figure the cost of new tires, Bearings needing repacked, brakes checked for operation, and the replacement cost of any of the mechanicals you can't get working (water heater, water pump, furnace, A/C, Convertor, vent fans, etc). Batteries will need to be replaced if over a couple years old.
The cost to polish a 29' Airstream by a reputable shop is in the $10k range.
Good luck with your sale. Advertise it on this site and wait for the phone to ring.