Welcome to the group! You will find that there is an incredible wealth of information available here from a friendly bunch of people.
I have the feeling that you have seen one available in your area and are trying to get an idea if what the owner is asking is fair. As in anything that isn't a "commodity" (such as Airstream trailers in relatively low population parts of the country), the answer you seek isn't something that's fixed in stone.
As an example, I live near Toronto, Ontario, a region with about 5,000,000 people and it took me about 6 months of regular searching to come across the unit I wound up buying. It wasn't the exact one I wanted, but the condition, size, and features that I deemed necessary were all there.
Because an Airstream trailer has many of the things a real home has, the condition of a 25 year old trailer such as the one you are considering is something that can really only be reliably checked by an expert. There are "amateur experts" available from the membership of this group in many parts of the country, if you wish help with the compexities involved.
By the way, I say "amateur" only because they don't charge for the service. The list is available from a link on the "Portal" page, I believe.
If you want a wild guess, I would hazard to say a low of $5,000 for the low end of the range of condition (things would NOT be perfect, one or more systems would be either missing or defective) to a high of maybe $10,000 for one that has been well-maintained, and in which all the systems (heat, hot water, electrical, axles, refridgerator, and so on) are present and in good condition.
Now, this is only one man's "guesstimate", so take it with a grain of salt.
If you want to discuss things to look for, please tell us more about the details of the one you are looking at. Photos are always a help, the more the merrier.
If it's to be, it's up to me.