Ah, the age old question... First, have a look at the following link:
Price vs. Condition - Vintage Airstream
Next, compare this trailer to others that have sold recently, that are in similar condition on ebay.
In my humble opinion, a trailer selling for that price ought to be completely road-ready. I don't mean that everything works, as much as everything has been worked over. It should not have the original axles, or AC, and most of the appliances will have been changed out too. There should be no floor rot, rear end separation, or general disgusting smell. If any of these exist, then the price should be reduced accordingly. If the floor is rotting, or the rear end is separating, then you are looking at a floor replacement, which is major surgery.
There are several checklists on the forums and possibly on the site above as well, that show you what to look for. Its tough to identify the condition of the floor if the previous owner has laid some kind of hardwood over the original subfloor, but you can identify rear end separation just by standing on the rear bumper and watching the lower trim for relative motion.
What I have observed is that 70's vintage trailers don't command a very high price, and "originality" doesn't much play into the equation. What is going to be worth money to you (ie. a premium price) is how much work the current owner has actually done to turn back the clock and make a 40 year old trailer useable (and safe). If all they have done is check out the systems, fix the leaking plumbing, and replace the shag carpet with pergo, then I would say their asking price is probably way high. I took a look at Craigslist for your area, and if the 78 Safari international is the one you are looking at, it looks to me like the owner has spent most his time shining up the exterior, and very little time with anything you will need to get you down the road. A shiny exterior is just icing on the cake--a pair of axles will run you around $1500.