The meaning of the phrase cost-effective in the first paragraph of this passage is C. getting the most for the money you spend.
If you insulate and seal air leaks in your home on time, you won't have to think about that in the future. Thus, if you pay a lot of money now, you will save a lot more in the future than if you didn't insulate, and then had to fix stuff around the house all the time, which would waste so much of your money.