The magazines alone are worth $250. Those steel Sterling-made 40-rounders go for at least $100 each, when they can be found. They have the reputation of being about the only 40-rounders that actually work (and they can be used in AR-15's as well as AR-180's).
As for the gun itself, the Sterling-manufactured version of the original AR-180 is the least desirable of the three versions. The most desirable is the Costa Mesa, California, made one; the second most desirable is the one made under license by Howa in Japan; and finally the one made under license by Sterling of England. These were also the order in which they were made in point of time. I have a Costa Mesa and a Howa (both of which I bought new), and I have to say that I wasn't tempted to add a Sterling to my collection at the time. I guess that typical British black paint finish put me off.
Value-wise, a Sterling (with one factory magazine and in pristine condition) should retail for about $1,000, a Howa for $1,200, and a Costa Mesa for $1,400 to $1,500. So you did very well on this deal.
The AR-180 found its niche market back in the day (the early 1970's) when Colt temporarily discontinued the AR-15 in order to redesign the hammer and bolt carrier to prevent hammer follow-down multiple firings if the disconnector was removed. In itself, the AR-180 is not as durable as the AR-15, despite having some design innovations such as the gas piston system and the side-folding stock. But the stock hinge is one of its weak points, as is the bolt hold-open catch.
Considering the lack of replacement parts, today the AR-180 should be considered a collector's item rather than something to be used for regular shooting. The bottom line is that the welded sheet-metal construction makes them fragile. Compare the Korean Daewoo K2, which is much sturdier and also uses a gas piston design. Too bad that that, too, is long discontinued.
