First it depends on how much you want to spend... Second the lowist your going to pay for a TRUE assault rifle is about $6,000 for an AC-556 (Fully automatic) Because semiautomatic rifles are not true assault rifles... Yes... even if they look like it! (Something the anti gun people love to coin is the word 'EVIL ASSAULT RIFLE" When they dont know what there talking about.
Well enough of that rant... Where do you live? you need to make sure what state you live in it's legal to own a rifle that takes over 10 rounds and has not made it cost an arm and a leg to get one.
You can expect to pay 3 plus change and up. The cheapest are the Romanian WASR's. I own a GPWASR 10 and it's very reliable. I also own an Arsenal SLR105(AK74). Arsenal makes great AK's and the WASR is the cheapest AK usually. The Arsenal cost me 6 and came with one mag and a nice muzzle brake and a other accessories. The WASR cost me 3 and came with two mags, mag pouch, slant cut muzzle brake, bayonet and other accessories. Hope that helps.
im personaly a Yugo M70 fan if your going to go with an underfolder thare the best and the barrels are mutch better if its ben upgraded with a zastva barrel an upgraded one will typicly run about $500-$600 its a damn fine gun.
well, not to try to change your choice of weaponry, but an sks can be used as a full auto weapon also, if relaxed your trigger finger will smack the trigger and empty the clip in the same time an ak does.