AK's can be had for under $500, in fact my local shop runs WASR 10 with 2 - 30rd mags for $399. AR's can even be found at or under the $700 mark, CMMG bargin bin complete flat top's for $599
CMMG Bargin Bin
The ammo for the AK will be cheaper, so if you plan to shoot it much you may want to consider that. Either one is fun to shoot.