The AR is the LEGO of the gun world. You need a bunch of specialized tools to work on the AC-556 and the Mini-14.
"Cheap" transferable Bushmaster M16 - $12,500. Transferable fixed-stock AC556 - $5,600. For the price of the M16, you can have two good quality rugged .223 machineguns and a DSA FAL para-folder .308.
Comparing an AR-15 to a Mini-14 is fair game. But, comparing an AC556 to an M16 is like comparing apples to oranges. What you are really comparing is a license to own a machinegun in multiple calibers and formats with a license to own a machinegun that is designed only to fire 5.56 (yeah, it can fire .22LR also).
If you need or want all of the calibers and formats that an M16 can provide, go ahead and pay the money for it. On the other hand, if what you really want is a 5.56 machinegun and don't care about other full-auto in other calibers, then the AC556 is quite sufficient.