Problem with AK is the cartridge (tapered case causes unique mag and feeding constraints, short range rainbow trajectory due to large bullet and small case), not the rifle if you get a good one. And good ones are Arsenal, Krebs, etc that run $1000+ and go with 5.45x39 and its just as capable as an AR.
The AR is a bargain. There really isnt anything else in that price range (500-600) unless you count parts kit Cetmes or bottom barrel WASR type AKs. Everything else AR quality is going to be $900+ for a FAL, PTR, etc. AR quality AKs cost more than ARs ironically.
Haven't looked at Saiga or Vepr AKs lately to know where they land. They should be a good quality AR alternative in the $500-600 range?
Generally don't expect any quality centerfire military pattern semi auto rifle outside the AK and AR for under $1000 give or take with the basic $900 ish PTR91/G3 being the next step up price wise for someone who already has a AR or AK.
On the other side of that coin you get into LWRC ARs, 308 AR10s, M1As at $1500 on up.