I am so sorry that your girl and you are going through this. You are doing the right thing, getting her right to the vet.
Each vet is different, so I don't know that anyone here can estimate the cost for you.
My female, Poppy, had a facial abscess this past year and we were fortunate enough to have it respond to medication and it did go away (it took a while - it did not go away as quickly as it arrived!). I do know that the first thing recommended is to have an x-ray taken to see if there is indeed tooth involvement. My vet used Clavamox and Covenia. Again, each case is different and what worked for my girl may not be appropriate for your girl.
Folks with more experience will be along shortly. Please keep us updated on what your vet says. Good luck!
Mom to 2 humans, wife to 1, -- and "mom" to 1 dog, 2 horses, 2 gigantic Red Oscars,
and 11 of the sweetest gliders!
http://highglass.wix.com/highglassgliders#"Constantly choosing the lesser of two evils is still choosing evil."