When is it time to put my rat to sleep?

Question
I have a female hooded rat who is just short of 2 years old. She developed a mammary tumor in her groin area about 6 months ago which has quickly grown in size. She has lost a lot of weight and is practically skin and bones with exception to her tumor. I took her to the vet a couple months back who said she was a poor candidate for surgery, and would probably not survive the removal. Since my vet is small town she referred me to a strictly exotic vet who quoted me $600 for the tumor removal. I opted to not have the tumor removed.
Since then I have begun to notice blood coming from her vaginal area on occasion. I have noticed her grooming her cagemates much more often than she grooms herself, and while they tend to return the favor she usually shoos them away when they reach her hind quarters, which she has a particularly hard time reaching herself. She has a noticeably strong odor that her cagemates do not have. Her leg has become completely enveloped in the tumor and she has started falling much more often because the tumor is in her way. She has begun to opt to sleep by herself in a box that is not big enough for her, instead of sleeping with the other two in a box that is big enough for the three of them. She is far less independent when she is outside of her cage, not taking the opportunity to explore as she used to but instead cuddling with me (this part I'm OK with :), however it is a large change in behavior.)
It makes me sad beyond belief to think her time may be here, but I would much rather put her to sleep than have her suffer or starve to death. She was the very beginning of my great rat owning adventure, which has been nothing but rewarding, and I want to do what is best for her.
So my question is, when is it time to let her go?

Answer
I am really sorry that the vet you saw told you that your girl would not survive the surgery. This makes me question the vets credentials. I am about as sure as I am sitting here that the vet you saw that was supposed to be an exotic vet was no more an exotic vet than Dr. Suess is. Maybe Dr.Suess knows more....that would not shock me any.   The ONLY reason your rat would have been turned down for surgery is if she was either too young, too old or too sickly to undergo anesthesia. There is no real reason for denying the surgery otherwise.  That high price was because I bet my left arm that vet knew you wouldnt go for it and decline the surgery....because I bet he sees few rats and he knew he would probably kill yours if he put her under.  Please go private and tell me the name of the vet. I am very curious about his credentials. If anything, he is just a general DVM that sees exotics as a special interest but has no real formal education or certification in exotic companion mammal medicine.  That irks me to no end thinking this could have been resolved a long time ago. It still can, but I fear if she is that weak from weight loss that it may be very risky. What I did with my rat, who was nearly  3 years old....she was a wild rat I had saved as a baby and she had numerous tumors that she went under anesthesia for twice and finally at the age of 2.5 I said enough surgery.  I felt so bad watching her waste away yet still have alot of zest for life in her.
Long story short, it was either put her to sleep or operate, so with just a 5% chance of surviving the surgery, we operated anyhow (it was worth taking the chance, otherwise she  would have been put down) she lived, though.....my strong brave baby girl made it through a bloody 90 minute surgery that was prbably harder on our Vet than it was on her!  LOL!  He was sweating bullets during surgery mostly because of the blood loss etc...  My Holly lived to be nearly 4 years old, with the surgery giving her nearly eight more months   which is a life time for rats.

You need to ask these questions......or answer them for me, rather.

Does your girl still want attention from you and still want held? Does she still seem to care about the things she used to care about, maybe not as much, but still reacts to them.  

Tell me about her diet and what type of  foods she eats.