Well from what I understand the vet will probably do a culture to check and see how severe it is and what meds to use. Typically they will prescribe Baytril, Fortaz, or Amakacin as an antibiotic. I asked in another thread a while back and someone said that in their area it's $30 for the visit, and around 30 for the meds depending on what is needed and how much. It might vary though based on your area and the vet. To help fight it though keep your temps high (but don't cook her) and that should help fire up her immune system.
It could have been that the previous owner either didn't know/care about the RI or needed to sell her in a pinch and didn't mention it on purpose. And btw I am bit of a newb here so take everything I just said in stride.