First AR built from parts (2002):
DPMS LoPro upper from MidwayUSA on sale $65 (ordered and marked as a "stripped A3" oops.)
DPMS 16" stainless bull barrel, (kit:FF tube, gas tube, etc) from Delton $210
RRA B/B/C (enhanced) & LPK from Pete-in-NH $175 shipped
Eagle Arms lower from Quantico Arms $106 with tax
Bushmaster A2 stock from Bealton gunshow in 2000 $55
JP reduced power spring kit $10 MidwayUSA
Barrel wrench $12, homemade wood barrel vise $0
Total: $626 for a near-exact copy of a DPMS LoPro Bull MSRP$850-ish, street price around $725+tax at the time.
All I've done to it is replace the reddot with a ACOG Ta-44 (also from a super-sale from MidwayUSA), switch to an A1 stock and add a Samco grip.
Its my IPSC gun and even shoots "that russian crap" nicely.
Two things you can't say about a rifle you built yourself, "who put that there?" and "what does this thing do?"
I love it and have built many more since.
BUT my cheapest AR:
Colt A1 upper with a 11.5+5.5 FH XM-177 lookalike barrel
Essential Arms lower, Aluminum CAR stock, Colt 30rnd mag
$400, with 700 rounds of LC on stripper clips in 1997.
Cheers, Otto