You need to be more consice about what you mean. "100sf" of gutter cable doesn't state how many linear feet are being used, which cable you used or what you pay per kilowatt hour for electricity. Post that information and I'll tell you the answers you are looking for.
Though there are a few different types, it is a cable. We use a self regulating cable that is mechanically attached to the bottom of the gutter (we install 2 runs, front and rear) and then a loop down each leader to the very bottom. This cable will adjust it's running temperature based on the ambient temperature around it. If snow is in contact with the cable, it will get warmer in that specific area than an area that is already clear and dry. The self regulating aspect makes the cable more energy effcient than the single temperature cables. This cable keeps ice from forming and allows any moisture in the gutter to flow to the ground.
Heating the roof is a whole different concept. We don't use a cable on the roof. We use Zmesh low voltage heating elements UNDER the shingle.