What I consider plinking/range/practice ammo, is any 50-70gr FMJ that I can get my hands on for around 40cents per round. That's for brass. I consider that to be sort of the vanilla price. Sometimes you can do better.
If you're willing to shoot non-brass Russian (I am in some cases), you might pay 30% less.
If you want to spend less still, you're going to have to begin investing in a reloading regimen that will eventually pay dividends over the long haul. And then you may have to buy your components in bulk to reap the full benefits. Without a good strategy that includes buying/shooting in bulk, 223/5.56 reloading (in order to save money) can be tricky.
I get my ammo all over the place. Much of it online...also from local sources, including Walmart (don't think I've bought anything there in nearly a year, however).