I've only seen Mossberg's MSRP's on their AR-ish rifles. Mossberg's AR's are missing a couple things that pretty much all "standard" AR's have so the street prices on their guns had better be below what's on the market now.
You can assemble an AR, using a complete upper, stripped lower and LPK for around $600. You could save a few more dollars by assembling the upper but you'll need some specialized tools.
Palmetto State Armory, Spikes, Rock River, Bravo Company, Del-Ton ... and a dozen or so others all source their stripped lowers from the same couple manufacturers.
Get one of those and put together your own lower. It'll be just as good, if not better, than the Mossberg of S&W budget guns and you'll learn a lot in the process.
Twist rates determine the rotation of the bullet as it travels down the barrel and then toward the target. A 1:9 rate is acceptable for most people. If you want to shoot heavier bullets - say 72 grain+ - you might want a barrel with a 1:7 twist. The faster rate, in theory, is capable of stabilizing a heavier bullet.
Chambered in 5.56 is good. That allows you to shoot .223 if you want. I wouldn't recommend going the other direction (shooting 5.56 is an upper chambered for .223).
BTW, if you assemble your own lower, it's chambering generally doesn't matter. Many lowers are marked "multi". This means you can build a gun, based on that lower, that will shoot 5.56, .223, 6.8, 7.62, etc.