While this M4 is nowhere near the cheapest, I did pretty well on the parts (prices from memory, but pretty close):
New (doesn't even have taper pin holes in the barrel, thus the PRI) Colt 14.5" M4 barrel "C MP 5.56 NATO 1/7 - $100
PRI flip sight - $125
DD 7.0 - $225
Aimpoint M2 with mount - $350
VLTOR buttstock - $135 complete
LMT BUIS - $80
JP stainles tactical bolt/carrier with bolt - $150
PRI Gas Buster - $65
SPR Grip (came in a large lot of stuff that I have since parted out for a profit) - now FREE
Bushy lower - $209 with LPK installed
Vortex FH - $50 (the only part I really didn't get a deal on)
For me the right place is the EE, and the right time is "almost always there".
Now that's a pretty large sum of money in that rifle, but look at the retail prices of all the items listed and I've saved A TON of money on this build. Saved me over $550, by my crude calcs. Sighted it in yesterday and it shoots great.
BTW, WTF is with these damn Aimpoint adjustment knobs!?!?!? WTF is "UP-Left/UP-Right"!?!?!? It took my forever to sight this in, just like last time. That's the second time I sighted this one in and I can't figure the damn things out.