I have never been fond of the 5.56, as a matter of fact my very first AR was in 6.8spc. Some years later I decided I should have a 5.56 and bought a DPMS, but shortly thereafter converted it to 6.8spc. I owned a Mini-14 years ago, but it didn't last long, just a personal aversion to the caliber at the time.

But now that I can go to the range more regularly I want to shoot the AR's, but at about .60 cents a round for 6.8, that is some expensive plinking. I found a complete lower for < $150 online and thought it would be an inexpensive way to get into a plinking AR. So with a complete barreled 16" 1:7 upper on the way as well I will soon have an AR in 5.56. At .30 cents a round it will be a much more affordable option and still be shooting from an AR platform.

I'm looking at some 20 round mags and a PA advanced micro-dot to put on it, I may even use it as my truck gun. And I guess I should have a 5.56 rifle in the battery should times get dangerous since it's such a wildly popular and available cartridge. Although I feel the 6.8 is a more effective round the 5.56 seems to be effective enough.

I can find steel cased ammo all day long pretty cheap, but am a little leery of shooting a lot of it, preferring to shoot brass cased ammo which seems to be slightly more expensive.

Yep, all reasons to own an AR...

If you buy all one headstamp of Brass cased ammo you may be able to subtract 3-5 cents off of the cost if you gather and re-sell the brass here.

I own 12 AR15's. 11 are 5.56. Only one is in another caliber; 224 Valkyrie.

My only aversion is for 6.5 Grendel. It does nothing special and the ammo is too expensive.

I don't see people taking some of the largest game in North America with 22cal bullets from an AR.

Match ammo for .223/5.56, 6.5G, 6.8, 224 Valkyrie, and others are very similarly priced. Nothing will be able to compete with 5.56 bulk/milspec ammo price wise.

Everyone needs a 5.56 AR though, IMO.

OP - why no .308?

You can get a lot of .308 ammo that is much cheaper than most 6.8 ammo you can find.

.308 gets you into a bigger heavier usually more expensive gun, I wanted to keep the AR platform that the girls could shoot too and not get turned off by the heavy gun and more recoil than 5.56/6.8/7.62x39.
I have a .30-06 if I need a bigger thumper.

When the ballon goes up, it'll be a 5.56 world all day/every day.  The most  likely  - and reliable -  platform to hose with will still be a decent AR and plenty of mags to feed it.

Everything else that goes 'bang!' will be secondary or of remote use, or will simply fail if run hard.