Our dog Augie had one of those 'fluid filled warts' a few months ago. When we took her to the vet he just popped it like a zit and she didn't seem to feel anything. (I'm not reccommending doing this in case it isnt). The Dr. told me it was called a 'hot spot' and lots of older dogs get them. I'm still a little fuzzy of exactly what it was but she hasn't had any since.
Good luck with the vet bills! I swear, it costs more for the animals to go to the vet then it does for us humans! (and that is saying something!)