Quality 5.56 surplus ammo is .30 a round, 7.62 is .50 a round. I think that's cheap ammo for rifles that cost over 1K. I've shot over 3k rounds of surplus 5.56, 7.62 and '06 this summer. Ammo cost is part of the hobby.
A good AR15 is about $800 and a good 7.62 AR is about $1500. You can go cheaper with the 7.62 AR but it may need tweaking.
The only malfunction with my AR10 has been from riding the charging handle, otherwise over 1K with out any problems over the past three years.
I'm a hoarder,
I have over 100 rifles, mostly mil-surp, the 7.62 AR is one of the nicest rifles you well ever handle. It's a AR15 on steroids. They have a large after market parts support now and more than a half dozen manufacturers to choose from.
Does anyone know about the Colt 901 modular carbine. With a conversion kit you can switch the .308 with a mil spec .223 upper. Also have any of you tried the .22lr conversion kits?
IMHO the 901 looked better on paper than in person. Swapping between calibers is much more than just pinning a different upper on the lower. Upper adapter, mag adapter, change the buffer and spring. The you have a overweight AR15. Two dedicated rifles makes more sense to me.
.22lr caliber AR's are boring.