That is a real classic Airstream. Overall condition and any added options or modifications is going to influence your pricing.
Begin your research with NADA Guides which provides blue book prices. You can also research comparable sales online to further investigate how much it is worth.
Here are instructions for finding a blue book value:
Go to [www.nadaguides.com]
Select = [RVS]
Select = [Travel Trailers/5th Wheels]
Select Manufacturer = [Airstream]
Select Year = 
Select Model = [Overlander]
Enter Zip Code = 
[Get Base Pricing]
Suggested list price = $12915
Low retail = $7450
Average retail = $9000