AR-180s vary a bit based on who made them.
Armalite made about 4000 of them in their own Costa Mesa workshop from 1969-72 along with about 1200 of the full auto AR-18. They were not really geared for mass production and many of them show signs of hand fitting. You be the judge if that's a good thing or a bad thing.
Howa also produced about 4000 AR-180s under license from 1970-1974, which ended when the Japanese government export restrictions forced them to stop.
Armalite eventually arranged for production by Sterling Armaments Company in Essex England. They made just over 12,000 of them from 1979-1985.
Armalite was sold to the Elisco Tool Company in 1983 and they had plans to produce the AR-18 in the Philippines as a replacement for the M-16. That's actually what Stoner envisioned when he designed it - a select fire service rifle that could be built on low tech equipment by countries that lacked the ability to make the much more complex M-16.
As a result the tooling was shipped from Sterling to the Philippines, but Elisco never got it into production.
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With that as background value wise the Costa Mesa and Howa made AR-180s bring a higher price and higher quality is often claimed - but I've never observed that to be the case. Sterling made examples are the most common and bring lower prices.
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40 round magazines are legal, and from time to time 40 round Mini 14 and AR-15 magazines have made brief appearances. Reliability is always problematic as it's a lot of rounds for the follower to lift and a lot of curve in the magazine.
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In terms of cash price, condition is everything if it's in excellent condition:
Howa or Costa Mesa with Colt scope and mount = $3500-$3700
Howa or Costa Mesa without scope and mount = $2200-$2500
Sterling without scope and mount = $1800-$2000.
Sterling with Colt scope and mount = $3000-$3200.
Original AR-180 steel magazines will bring around $90 with the 40 rounders bringing a little more at around $120. I've seen regular 20 round magazines sell as high at $160, but that was an outlier, with bidders getting to focused on winning an auction.
Again those are prices for "excellent" condition examples. If it's just very good, expect to get at least $500 less.