Gus,
By way of HK 91 Clones (by your definition, the HK 91 is a clone of the full auto G3), there are several choices:
Fed. Arms Aluminum receiver guns. $600
Hesse Steel receiver guns $600
FMP Steel receiver guns $1000 **
SW-3 Steel (Cast) metal guns $1200
SAR-8 Aluminum receiver guns $1200
SAR-8 Greek Steel receiver guns $1300 **
By way of desirablility, the FMP guns (FMP is Portugese, and a licensed contractor for HK) are the best post ban guns with a pistol grip.
The Greek SAR-8's have thumbholes (Hellenic Arms is a licensed HK contractor), but can get enough US parts to use a pistol grip.
The SAR-8 with the aluminum receiver is basically the same gun as the Fed. Arms gun, but with a nicer parts kit, and the Springfield lifetime warrantee behind it. It is supposedly better fitted and finished.
The SW-3 uses a cast stainless receiver, and are supposedly nice guns (more trouble free than their SW-5's, especially the early ones).
Special Weapons (SW) sold a bunch of their receivers to Century, who uses them in their Cetme guns (basically, an early version of the HK 91--the Germans used the Spanish design almost in toto for their guns). These are supposedly good guns, use the HK mags, and run $600ish.
I forgot one--the Hesse--it looks like an HK because it is sheet metal, but then a Yugo looks like a Mustang because they both have 4 wheels and 2 doors (need I say more??--look for reports of Hesse AR's around here).