Seems like there are different concentrations, but I recall somewhere around $60/qt that makes quite a few gallons of treatment. Also I remember being advised it would take two quarts for a typical home.
'Do my own' wants ~$30 for a quart or ~$55/gal of late for 7.9% concentration. I've bought quarts and they last a few years but the gallons are far more economical. Bifen IT Shelf life has never been a issue for me.
FWIW I used 2 qts of Bifen-IT in 55 gal of water and sprayed approx 300 perimeter feet. Not quite 2 gal of mixed solution for 10 linear feet. (and I used about an 8-9% solution. recommendation is 6%. up to 12% if you have an infestation)
I buy it by the gallon, and then spray the yard with it too, as it prevents ants, army worms, grubs, and a host of other pests.
The Termidor or Taurus is a whole different type of pesticide than the Bifen products. I believe Bifen kills quickly (I've used it for barn stall fly control). Termidor and Taurus are designed to specifically control termites by getting in the termites waxy coating and being tracked back to the colony, exposing all of them and killing the colony. Full termite colony control takes 90 days. They also provide termite control for up to 10 years.
I've been able to buy Bifen, Termidor SC and Taurus SC in Texas, through Do My Own website and they were also available through my local feed and seed store. Do My Own was half the cost of the Seed and Feed store. You might be limited on quantities though. I think that's the case with 2,4,D anyway, at least at Tractor Supply. You can buy quarts, but not gallons.