Short answer: Your aluminum foil lid is the highest, best ROW (return on weight), followed by (other pot attributes being equal) heat exchanger fins on your pot, and lastly, a cozy.
Long answer: For the cozy, it depends on your cooking habits and trip length. But as an example, I'll make some assumptions and do the calculations:
1 liter pot, 0.5 square feet of surface area on the sides.
Delta T between pot temp and ambient air: 190F (average of 170 to 212) minus, say, 50F = 140F
R-value of air film with no cozy: 1
R-value of cozy plus air film: 2.5
Conductive/convective heat losses to the air: 0.5 sq ft x 140 F x 1 (BTU/hour-sq ft - F) = 70 BTU/hour heat loss.
Two meals a day, hot drink and hot entree in each, two weeks, 10 minutes heating and eating each of the four hot items each day: 4/day x 10 minutes each x 14 days = 9 hours.
So 630 BTUs saved. butane, propane and white gas are about 20,000 BTU per pound (but a HX pot is about 50% efficient, so you only get 10,000 BTU per pound of fuel) so 1 ounce of fuel is saved by using the cozy.
Without showing all the details for radiant heat loss, I estimated the cozy outer temp at 106F versus the bare pot at 190F. The difference in heat loss (153 BTU/hr off the bare sides versus 88 BTU/hour off the cozy) gives a savings another 585 BTU over 9 hours or one more ounce of fuel for a total of 2 ounces of fuel saved (conduction plus radiant).
Bottom line: You'd need to be be doing a fair bit of cooking over a two week period to come out ahead, balancing the weight of the cozy to the weight of the saved fuel.
However, not burning your hands on your morning coffee and oatmeal probably has some value. One pot of Ramen dumped onto to the ground because you were burning your fingers, and that represents weight and time. If you didn't have a cozy, would you use a pot gripper? I've made a 9 grams pot gripper, but most commercial ones weigh more.