I have had plenty of snakes with upper respiratory infections come through here, as far as rescues or even buying them because I felt bad and knew they would die if I did not. Your under the tank heat mat should be on all the time any how, if you need to leave the heat lamps on that is fine, just try not to use white light as it could stress them out, like I said ceramic heat emitters, or red or black heat bulbs are best.
Out of all of the snakes that I had treated for respiratory infections, the highest bill I had was $200, and the lowest bill I had was $90, so I would expect it to be any where in there. Usually you will be prescribed Baytril injections. How ever, my vet did tell me if it were a mild case and if I wanted to try and save money, you could buy Saline Nose Drops and a needless syringe that reads ml. and you would use 1 ml. and shoot it up their nose, 1ml. per nostril. Just make sure you shoot it in their nose and not their heat pits. Granted, I never did try it, I just went in to the vet's office, but I do trust my vet and she is very good, so I guess if money is tight that could be an option if it is mild case and not severe.