For a time, Fulton offered complete retro rifles and uppers. One was the Hornet, which I had some of. The other was the Legacy IIRC. They were pretty standard SP1-ish types, and used Saco 1/12 chrome lined barrels. During the ban period, they used a faux flash hider muzzle device that resembled an A1-type. After the ban, it was the standard A1. Fulton used (and may still use) an awful lot of DPMS parts, but applied better assembly and quality control over them. The Legacy and Hornets I had and saw ran pretty well, due in no small part to the gentler 20" gas system. They can be a fun plinker, especially if you're into retro style guns.
If it's one of those, I'd peg the value at somewhere between $250.00 and $350.00 depending on the variant and market. If you have something else, all bets are off without more info.
2012 National Zumba Endurance Champion
Hi Point 9mm, Lorcin .22, Bushmaster Carbon 15, Norinco SKS, Leapers 10x