You would need to raise the water temperature to 40C/104F and hold it there for 1hr in order to disinfect the tank (
source and
source).
Salt varies by brand, of course, but if we assume about $0.45/gallon, the cost of a single 10gal transfer is $4.50.
Heating a gallon of saltwater from 80F to 104F would take about 210Btu's at 100% efficiency. I don't know of any aquarium heater that will get to 104F, so we can cost out removing the water and boiling it in a pot or something. There's a lot of variation out there, but a typical gas cooktop will have around 30-40% efficiency in most cases. So we're looking at about 5250Btu's (or 0.0525 therms) of energy to heat 10gal of water. Natural gas generally costs around $1.20/therm after taxes and fees and such, so we're talking ~$0.06 to heat the water. Holding water is a lot harder to figure out due to all the variables involved, but we can assume that heating and holding 10gal of water for an hour would cost less than mixing up new salt. But hold on - that's just the water! The tank itself still needs to be disinfected. You can't just pour the water into the tank and let it sit, because it wouldn't hold at the required temp for the full hour...
So you'd have to look into something in the tank to keep the water at the proper temperature. Something like an immersion cooker, perhaps? That could work, but now you're talking about a special piece of equipment that has a not insignificant cost - not to mention that electricity is more expensive than natural gas. I can't calculate an exact number, but even assuming we leave out the cost of the equipment it's not going to be too far off from that $4.50 number for the salt.
In the end; we're talking about roughly $20 in cost if you're using a 10gal tank for TTM (which most people do).
You can cut this cost in half if you use a 5gal bucket instead of a 10gal tank - which seems a far easier way to save some money to me (not to mention the $20+ you save by not buying the tanks).
All that being said, I don't think the salt cost is the main reason people don't do TTM. The most common reason I've seen is doubt in it's effectiveness, or else the space/effort involved in so many moves. There's also the "stress on the fish" argument, but that's generally against QT in general (since copper/CP treatments also cause stress). If any of those arguments come up, it's generally because the aquarists isn't really sold on (or interested in) doing QT - and that's a different conversation than how to save a few dollars...
