These are exactly the questions and analysis I'm running into in the final phases of unnecessarily tweaking my mostly-CDT gear for the PCT this year.
Our sleeping bags (a pair of WM DryLoft Antelopes) are 2lbs 10oz each, but we're not in a position to spend $740 ($370 x2) for a pair of Ultralights anyway. $740 to save 26oz would cost us $28.46/oz.
The best we've done so far: replaced our ClikStand proto (3.3oz) with a MYOG hardware cloth pot stand (0.7oz). 2.6oz saved for $3, or $1.15/oz.
Almost as good:
trusty Tyvek groundsheet: 7oz.
Polycryo: 2oz.
5oz saved for $8, or $1.60oz.
The other cost-efficient weight savings for this hike was replacing my 8.0oz ZRest (which needed replacing anyway) with a 3.5oz GG TorsoLight. $16/4.5oz saved is $3.55/oz.
CDT Marmot Precip: ~12oz.
DriDucks: ~6oz.
~6oz saved for $15 = $2.50/oz. Also, I was tired of the poor, non-waterproof, non-breathable performance of the Precip so I had to do something about it anyway. As a bonus, the $15 included free pants that I will never use.
Replacing my trusty vintage Patagucc Puffball vest (8.1oz) with a BPL Cocoon Vest from the Gear Swap (5.5oz, $60) isn't nearly as good a payoff: $23.07 per oz saved, but the Cocoon is at least 2x as lofty as the packed-out Patty and was cheaper than buying the Montbell UL Parka I was lusting over.
ETA: Wife:
MHW Down Jacket: 25oz.
WM Flight 10.5oz, $140
$14.73/oz saved. Hmmm, it seemed so much better than that at the time. Perhaps it was the absolute amount of weight saved, or how much she likes the new jacket.
This type of analysis is what keeps me from coughing up $15 for a Ti spoon or $70 for windpants that are 3oz lighter than my $4 thrift-store Nike wind pants. I'll be keeping en eye on this thread for last minute bang-for-the-buck ideas.