Poor Jasmine !

I thought it was going to be a normal afternoon that day,but it wasn't. Mum had just cleaned out the mouse cage and I went to check on the mice. I lifted Jasmine out of her cage to find that she had a lump a bit smaller than her head between her left back and front paws. About ten minutes later we took her to the vet. He said that it could be a tumor or maybe even cancer but it was too early to tell at this stage. I asked when I should bring her back and he said ''If the lump starts getting so big that she has trouble walking or if it comes through the skin and bleeds. The surgery to get it removed would cost $100. She is about one and a half years old. The vet also said that any mouse that lives beyond two years is quite lucky to be alive. If the lump gets worse I don't don't know if I should get her put down or get her the surgery. What do you think I should do ??