DSS, 6.8 ammo is almost as cheap as the others. I can get SSA and Hornady ammo in several different flavors for under $20 a box.
Hunting ammo is about the same, but cheap plinking ammo is non existant for these. I like 6.8, but recommend it as a second AR. Not a first AR, unless you have deep pockets for plinking. Ammo for 223 is in freefall, you can get 1k rounds for 215 if you look. I can shoot my 308 AR for the 50 cents a round which is what 6.8 is. The big 6.8 advantage is that it is awesome to SBR. Several hunting guides I know use SBR'd 6.8's so the silencer doesn't make the gun as long. 6.8 doesn't require a long barrel to burn its powder like some other rounds do.
223 VS 6.8 comparision
Hornady 75 grain match BTHP 223
Muzzle 100 200 300 400 500
2790/1296 2561/1092 2344/915 2137/760 1941/627 1757/514
6.8 Hornady match
Muzzle 100 200 300 400 500
2550/1588 2313/1306 2088/1065 1877/860 1680/689 1500/550
Right around 200ish pounds more energy for first couple hundred yards. The 308 is 1000 pounds more energy at every distance over 6.8, and cost is same for ammo.