$900 for gun + mags. Not for ammo. Although I am eyeing the 5.45, 5.56, 7.62x39, and 7.62x51 because of their bulk pricing!
5.45x39 is the cheapest bulk you can buy but the problem being is when the surplus runs out... And it will. .223 vs 7.62x39 is comparable for steel case only. Brass .223 or different bullet types like M855 are substantially more than steel case. Surplus 5.45x39 is steel core which will give you problems at a lot of ranges.
Pick a caliber, then pick a gun.