I'll 'ditto' YuppieJr's recommendation on Wolf Gold, if you search around you should be able to get it at/under .30/round shipped at 1k quantities.
I am not adverse to using bimetal(steel case) - and have found the 62 gr variety shoots very accurately. However, most ranges and competitions ban anything that attracts a magnet regardless of whether or not it is steel core(SS109/XM855). Lucky Gunner did a great article on brass/steel case here:
http://www.luckygunner.com/labs/brass-v ... ased-ammo/For shooting - I am not a fan of the XM855. It just doesn't group as well as M193 for me. However, like I said, I've had good luck with the non-penetrator 62 gr. As others have stated, it will probably go up in value at some point. But again - not allowed at most ranges/competitions.
As far as an accurate bullet, dollar for dollar I find Fiocchi 40 and 50gr V-Max in .223 to be an exceptional value when I can find it at ~.40/round. Actually anytime the Hornady V-max bullet shows up at under .50/round regardless of manufacturer I will generally pick it up. They say that the 40gr won't work in a 1:9 or 1:7 twist, but I've had good luck - of course every gun is different.
If you are going to hoard NATO stuff with the intent of using it double and triple check it will work in your gun - like running several hundred thru it and verifying that your not blowing primers, hard extraction, or anything else. Just because the barrel is marked 5.56 doesn't mean that the tooling wasn't worn by the time your barrel came thru.
Stock up on _good_ magazines too.