i use bushmaster's every day at work and hold up so much better than the other ones we had. but if it was for me i think i would go with a rock river arms or colt. if i was building one i always use rock river arms lowers but change the trigger.
You pretty much can't go wrong with any suggestions here.
As far as lowest cost for a good quality production gun, I say Bushmaster, DPMS, or Rock River would all be good for a reasonable cost.
However, if you are going to tweak and alter it extensively, it may make more sense to build one up.
I have a Bushmaster M4 type with izzy flash hider; very well built with relatively decent price. Won't go wrong with this one. Too bad I don't know how to post a pic otherwise I would definitely show off a bit.
I am in the planning stage of building one of these. I want a 16" barrel but don't care for the shorty handguards. It needlessly shortens the sight radius and the barrel sticking way out there looks silly to me. So it's mid-length handguards for me.
Stag Arms forged lowers are made in the same plant as the Rock River Arms forged lowers. I just picked-up a Stag lower at the local gun show for $100 out the door (I should've bought two, but I can get another at the next show). All of the RRA lowers I saw were $140-160... Same damned thing!