Wolf = cheapest to shoot.
Old Wolf ammo had a laquer coating that could gum up your gun once it gets up to operational temperatures; this coating is not present on new Wolf production ammo. Most everyone agrees Wolf leaves more residue and dirties up your gun faster. People argue back and forth if the steel casing is bad for your ejector. Some people swear by Wolf as plinking ammo; some people won't touch it with your gun.
Federal Lake City XMxxxPD loose packaged ammo=second cheapest to shoot.
Milspec stuff supposedly. Some people argue that you'll get squib loads now and then. More people suggest you hand-sort the stuff and pick out any "questionables". I have shot half a box of 500 XM193PD flawlessly after picking out 29 "questionables". People have shot entire 1000 round boxes without handsorting, with no problems.
Remington UMC= cheap to shoot.
They're usually selling 55gr 200 round boxes for around $40 a shot, individually boxed. I shot a box with no problems.
Remanufactuered Black Hills stuff is supposed to be available at a huge discount compared to their premium stuff, I've heard that the cheaper stuff is equal in quality to the premium stuff. Black Hills has always been associated with quality.
Also worth mention is Winchester makes M193 stuff that is...Q3131A? Something to that extent, milspec too.
If you shoot out a barrel, drop in a new one. You can do it yourself or get a gunsmith to do it. Bare minimum DIY, all you really need is a multi-wrench with barrel lugs that can remove the barrel nut. You can purhase the receiver blocks, handguard removal tool, and other AR-specific tools, but you can get by if you're crafty and good at mickeymousing things.