First question is how large is the snake in question the not eating trick is a game for adults small snakes don't play it. How is your husbandry? especially your ambient air temps?

I would second the vet. Mucus is not something you should find. I would suggest having a culture done this will tell you if it is RI and in the event of RI it will tell the vet what meds will work best. This should be done before meds are given after it is close to useless.

Vets visits vary greatly. I can't tell you what your vet will charge but my vet charges about 70$ for a hour hour consultation with the vet and tech, the culture is about 45$ on top the meds are cheap so I get prepared injections (pre loaded) for less than a dollar a shot.