Unfortunately the cost of medical care isn't based on the cost of whatever animal needs help...a $5 rat, a $10 snake or a $500 dog, it matters not at all. You're
paying for the education, skills, medical care & "over-head" (including the staff, building & utilities) of the vet whose help you seek.

It's encouraging that he/she is still "active, hungry and very strong" but I don't know what to tell you...seems to be a very resistant infection. Was
there lab work done to identify the culprit you're dealing with? Poor snake...