I'm so sorry to hear that your little girl is sick. Its hard to watch them grow old and sick, and its not fair that our pets' lives are so much shorter than our own. I'm sure you've given her a wonderful life! *hugs*
As long as she's eating and comfortable, I think its reasonable to keep her going. When she's not enjoying life anymore, you'll know.
As an FYI, most vets around here will euthanize pocket pets and exotics even if they don't treat them. This may also be the case where you live. A few phone calls will help you find out.
Euthanasia solution is an expensive drug. And I know that price/cost of living is different in different areas... but $200 seems exorbitant. A regular vet (not exotic) will likely be less expensive. See above. Also, if you're sure its time for her to go, you can ask that there be no exam, just euthanasia. If the vet will do that, this will save you money, too.
Also FYI, the vet's method of euthanizing rats can vary widely. This might be gruesome, so prepare yourself for this next part. But you need to know. Most euthanasia solution is pentobarbitol. This is a barbituate, but also an anesthetic. So euthanasia with this drug is basically an overdose of anesthetic- they fall asleep like they were going out for surgery, but then they keep going. This drug is considered the only humane drug to use.
Again, this part is gruesome, I'm sorry... Euthanasia in rats can be accomplished either by injecting the euthanasia solution directly into the heart, or into the abdomen. If it is accurately injected into the heart it is instantaneous. If it is injected into the abdomen, they slowly and peacefully go to sleep. If it were my pet, I would make sure they "mask down" my rat with a gaseous anesthetic (like isofluorane) before they made the injection.
If you wanted to be with your rat when she passed, you could request the mask down and abdominal injection. Then they could bring her to you to hold as she passed. If you did not want to be with her, I would request the mask down and cardiac injection.
Again, I'm very sorry you're going through this. My thoughts are with you!
"My dither fish need dither fish!"