5.45x39 is, if you dont mind an ak74
As far as ballistics they are about the same but while the AK74 wont rival the AR15 as a platform, ammo is about $125 for 1260 rds
Alantic has AK74s cheap. Surplus 5.45x39 is really about the cheapest ammo out there besides .22LR.
http://www.atlanticfirearms.com/storeproduct911.aspx