Wow, a late 80's 23 footer. I've never seen one, but according to the Airstream Web site they did make 'em in 1987 when it was the shortest model available. If the dinette looks original, I can only guess that the floor plan might be similar to the early 90's 21' Sovereigns, with the dinette up front, a kitchen in the middle and a corner bed and bath at the rear.
On the other hand, since there is some confusion on the way Airstream measures trailers, I suppose it could also be a 25 footer. These have front couchs, mid kitches, mid bath's, and rear bedrooms. If you included the tongue and bumper in the 23', it's a 23 footer, if not, it's probably a 25 footer. Either way it's a desireable Airstream. As Kevin noted, the shorter late 80's Airstreams seem to be quite in demand and fetch almost as much as the longer models.
So what are they worth? I've watched prices for the last several months leading up to the purchase of our '85 25' Soveregn and since then, too. My opinion is that pretty much regardless of length, late 80's Airstreams are generally offered for sale at prices between $9K and $15K if from private individuals - a little higher if from dealers. I have no idea of the final purchase prices, but I doubt they are too much lower. You will occasionally find some offered above and below this range. More than age, what determines their values are condition and options. Late 80's Airstreams below $10K probably need a little work, are untested, and/or don't have too many options. I'll assume but can't really prove that those priced higher don't needed as much work and can prove everything works. I have noticed that higher priced Airstreams often are optioned with several of the following: window awnings, rock guards, solar panels, microwaves, special hitches, etc.
Now with that in mind, only you can determine the true value to you. Is the size right for your expected pattern of usage (i.e. weekenders, multi-week vacations, or "full timing"). Is the condition of the trailer right for you (perfect, or needing a little work). Are the options right for you.
As far as conditon, there are many message strings on this web site as to what to look for, so I won't go into that. An 80's Airstream is old enough to have many "needs" if not cared for properly, but many are in great shape. Good Luck.
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