A genuine HK91 in reasonable condition is worth between $1500 and $2500 these days, depending on accessories and condition. If you can find one for under $2000, count yourself lucky. If you find one for under $1500, count yourself extremely lucky and buy the rifle at once.
HK91/G3 clones come in varying qualities and prices, but even the top-of-the-line clone (Greek contract receiver Springfield SAR-8) shouldn't cost much more than $1200-ish.
G3 clones with cast alloy receivers, and CETME rifles made from parts kits and US-made new receivers are significantly less than $1000.