AR-180/AR-18...
treedawg...
I've seen conversion guns going for between $4.5K - $5.5K & C&R Costa Mesa AR-18s from $5.5K - $13K for the OEM shorty!
I bought a n.i.b. C&R gun with the Armalite Q.D. 2.75x scope for $2,275 in '96. These rifles are highly underrated! They are extremely accurate & function well in sub-zero conditions... unlike the M-16. Some of the high tech assault rifles coming out these days have a gas system patterned after this weapon. The only weaknesses are in the light duty folding stock hinge & a somewhat fragile bolt-hold open catch (this usually isn't a problem unless the operator were to dry fire the gun with the upper receiver hinged in the open position). The Sterling, steel 20, 30 & 40rd mags are superb... but quite expensive... ranging from $40 - $175/ea.
These weapons are appreciating quite rapidly... go for it if you get a deal!