The bill may not be accurate though either.
I know my vet hospital would discount procedures for rescue groups, so they most likely are getting one there as well.
Some of the numbers I've been dealt or had given others for similar procedures before:
A quick breakdown though is probably around 2-5k depending on some factors for the average person.
X-rays would probably run around 175-300$ depending on how many views are needed.
Meds can run up to 300$ depending on type, how often and so on...
600$ for blood work and culture of the discharge, unless they really had to run some extensive tests... Then more.
The surgery is really the big ?? here. If it was an abscess only? Probably not too bad all. It would be a bit worse than an average abscess because of the amount of necrotic tissue to remove and upping the length of anesthesia would increase costs. If they did have to do any type of repair to the bone then that would be more costly.
Drain removal isn't usually charged for unless the animal needs to be sedated for it. Considered part of the surgery recheck.