Math Help - Price Matrix/Per Degree of Insulation or Weight

I'm shopping for new camping gear, and there are two things that come up frequently when shopping. Price Per Ounce (is what I'm paying worth the weight decrease) or Price Per Unit of Warmth (is this warmer sleeping bag worth the extra cost, or should I buy two cheap ones and double them up?)

I'm a bit confused as to how to set up such matrices in Excel, mostly because in the case of degrees of warmth, how do you assign a value to something that keep your warmer in colder temperatures? Could I use the weight of the down as an indicator? More down = more warmth?

Weight is relatively easy because you can divide weight by a dollars and get a price per ounce.

You can just assign a 'warmth factor'. like say you need to stay at 100 deg (98.6 or whatever), and the 30 degree bag is then 100-30 = 70 warmth factor. The -20 degree bag is 100 - -20 = 120 warmth factor.

Something like that.

It will be hard though because I'm not sure two 30 degree sleeping bags actually keep you warm at -40. (70 warmth factor x 2 = 140 = 100 - -40).

I'll chime back in with a broader recommendation though (as an eagle scout and someone who has spent a fair bit of time camping/backpacking):

1. Pick your situation, and buy gear aimed at that. (if you will be camping in 20 degree nights coldest, grab a 10 degree bag - I grab a 0 because I hate trying to sleep cold). You don't want to try to 'nest' sleeping bags. Even liners that are built for it are a mess.
2. Low weight is for bragging rights. And you pay a lot for bragging rights in any hobby (cars, guns, fishing, camping, etc). You don't need a stove that is 2 oz lighter than the other stove. You do need one fewer fuel containers in your pack. What is the fuel burn rate of the stoves?
3. Heavier gear also tends to last longer than lighter gear.
4. The heaviest gear is the gear you do not use. I dunno how many people I see carrying multiple pocket knives, shoes, underwear, etc.
5. Spend money on quality for a tent, bag, and boots. They will last virtually forever. Everything else you can get by with cheap stuff.

You want math? You got it:

1. Sleeping person gives off 200-250 btu/hr/person of sensible heat, lets use 200 btu/hr
2. Sleeping bag rating 30°F
3. Surface Area of sleeping bag (a guess) 72"x36" times 2 for front and back: 36 square feet
4. Skin temp of 92°F for comfort
5. Assume down/bag thickness of 1" for simplicity
6. Insulation R-Value has units of °F•ft^2•hr/(Btu•in)
7. (92°-30°)•36Ft^2/(200 btu/hr • 1 in)
8. Gives us an R-value of 11.16