Not throwing rocks, the 18 barrel doesn't help much on a model or value.
CMMG sells two rifles with 18 inch barrels in 308, if it's factory, both are above $1500 for retail.
Street price is going to be around $1300-1400, and figure some less for used, probably $1000-1200 if you can get a buyer. The AR market is still pretty well saturated in my area, the .308s aren't moving. YMMV.
IMHO, CMMG doesn't have the name brand associated with it, they tend to fall in with Stag Arms, Rock River Arms, folks have either heard of them or they do not know they exist.
CMMG also sells a truck load of parts for ARs, you would need to make sure that it's not from a CMMG labeled stripped lower, with a bunch of separate parts to get the full value.
I've used CMMG parts in builds, they have been good, but I have also seen parts kits from them that were short pieces out of the box.
IMHO, they have a good reputation for quality parts and decent QC on the items they sell. I have used one of their barrels, it was a decent barrel in fit, finish, and assembly. I didn't test the accuracy, but it functioned as expected.
Last edited by AH64guy; 01/11/16.