First. Why would you agree to have the store that sold you sick snakes, that is rumored to sell sick snakes, treat your sick snake? Fighting fire with fire are we?
Second, not all RI's are created equal. Are you even sure that they are using an anti-biotic on the snake? Most effective anti-biotics require prescriptions. Baytril (enroflaxicin) indeed does require a prescription from a Vet. They are not a vets office, so why are they treating animals like they are? Many medicines will put tremendous strain on the animals liver and can damage them permanently if given improperly.
I would not continue to take this snake back to the store that sold it to you sick. That will not solve the problem, otherwise you wouldnt have been sold a sick snake to begin with. If they had all the means necessary to treat him before, they should have.
Either you can return the snake and again demand your money back to later on buy a healthy snake from a good breeder, or keep it and dish out the money to have a qualified herp vet do a culture to figure out EXACTLY what your are dealing with. Be it viral or bacterial. The vet will prescribed hopefully injections of medication, although oral are also common.
Sorry you think $120 is a lot for an animal you didn't even pay $50 for, but there is no way around that. Animals that are sick need medical care, and RARELY is it less then the cost of the animal.
Would you deny a puppy the necessary vaccines and fecals if you got him for free, even though the visit and shots will cost quite a bit?
It is part of being a responsible pet owner, and should be expected.
It's up to you. Like I said, call a vet, spend the money, have a healthy BP. Or take him back. I wouldnt trust the pet store to heal this BP, and more likely than not make the situation worse.