If you like it, that's all that really matters.
I took a look on Campmor.com to see what sorts of weights and prices were out there for some -15 F bags. They had a Eureka -15 F bag
for under 60 bucks that weighs about 6.5 lbs. Combine that with a wide of CCF pad
and "in theory" you can make it to about 0 F for under 8 lbs and $100 bucks. At those temps, I would rather not gamble on saving money. At 20 F, you can have a cold miserable night with gear failure. Below 0, "miserable" is more likely to be replaced with "dead".
FWIW, I believe the 12.5 lbs for the 4 part system includes a stuff sack and a compression bag. Those would have to be heavy though to make much of a difference.