Unless you're rolling your own, ammo is pretty much the same price per round for .223/5.56 as it is for handguns. Only ammo I can think that might be cheaper from the factory is 9mm. I'm thinking .40 and .45 is anywhere from 36-45cents/roundI own a bunch of handguns. I own a .22LR rifle. But I don't own an AR. Can't afford to shoot it.
I did just buy an "AR like" 9mm carbine so I have something to have fun with.