Ok guys, I need your help.  The end state goal is to build a rifle that is as close to my issue rifle as is possible on the civilian side.  Hopefully a colt m4 6920 with a 14.5" barrel, pinned and welded, with a KAC ras, an acog, a matech buis, a grip pod.

My question is- would it be cheaper to:

1) buy the 6920 "socom" model and have the barrel cut to 14.5 and pinned?
or
2) buy the 6920 "OEM1" basics model and add furniture and cut the barrel to 14.5 and pinned?
or
3) say screw it and buy a stripped colt lower, a random lower parts kit, and a 14.5 BCM or other quality upper already pinned and welded? (all i'm really after with the colt thing is the rollmark that says m4 carbine)
or
4) any of those options, but instead of cutting the barrel down to 14.5, just pull the barrel, sell it as a never fired colt 16" barrel, take the proceeds and buy a 14.5" barrel and install it?
or
5) just pay the tax stamp and get a 6921

any help would be appreciated.

I'd do (and have done) option 4. Had an A2x pinned and welded to the 14.5" barrel.
You could always do this: http://www.specializedarmament.com/content/skins/flat/14-5_bbl_16_bbl.htm

He's doing one of my Colt 6940 uppers now.

Cheapest?

1,4,2,3,5.

1:  Easiest to do; buy it and get the barrel bobbed to 14.7 or 14.5 w/an A2X.
4:  Same cost as Option 1 but even though you will recoup some of the cost by selling the 16" but will probably pay more to get a Colt M4/A1 barrel.
2:  The Colt OEM models are not cost effective.
3:  You will have a hard time finding a stripped Colt lower; complete Colt lowers are also not cost effective.
5:  This route will require the factory SBR route which, while doable, isn't really time nor cost effective (even w/o the $200 stamp and wait time).

IF you want to get something as close as possible, just bite the NFA bullet and build an SBR.

Buy a 6920 - 16" rifle from any of the hundred places selling them for under $850 and send the barrel off to ADCO and have them cut it down while you're waiting on your form 1 stamp.