I haven't checked, but I wonder what it would cost to take a box of parts to a gunsmith and have him assemble them into a rifle. What I'm saying is that non wear (or slow wear) parts can be purchased used. Say $100 for an upper and lower set. $250 for a Bushmaster barrel assembly, with handguards, frontsight base and gas tube with barrel nut. $125 for a good BCG. Another what, $60 for a DPMS lower parts kit. $100 for an MOE butt stock kit, including the buffer spring, milspec tube and buffer. Where are we now, just over $600 for a solid rifle? If you can find a used barrel assembly, you'd save more.
If budget is a problem, I'd be scouring the net for used parts. There are somethings that you can buy used and not have to worry about it. My biggest concern would be the barrel. Well, not necessarily the barrel, but the gas port. Some manufacturers like to make the port a little too big so that they will function inspite of some shortcomings. If going low cost on the barrel, I'd at least stick with RRA or Bushmaster.
With this build, you can add as you go. If you decide that you want to put a longer free float rail on it, all you have to do is take a hack saw to the front sight base to fit it under the rail and then get a flip up front sight to go on top of the rail.