Here is how I built my Rifle:
CMT A3 upper, LPK, charging handle, and bolt/carrier assembly from
Denny's Guns - $95 for the upper
$124 for the Bolt/Carrier assembly
$63 for the LPK
$17 for the charging handle
He ships for real cheap, and you'll have the stuff in about 3-4 days (from MO)
He also has M4 butstock kits for $78 - you might be able to to better at a show, or on the EE
Superior Lower from Sully - $98 including tax
Bushmaster barrel assembly
here - about $247 shipped - I got the M4 profile Dissy and it delivered in about 3 weeks.
That would put you at about $645. You would still need a rear sight of some sort, and a butstock of your choice. I have a Sully stock, and really like it, but they are about $120. You would likely be slightly over $700 when all done, but you would have a rifle built with quality parts. The only reason for it to not function when you are done is improper assembly. I only have about 100 rounds thru the one I built (with some help from Death451
) but I have not had one failure. That's more than I can say about my first AR, which was factory assembled.