What to do about rat tumor, about to go on a trip?

I have a rat that I adopted in May from our local animal shelter (along with a friend we introduced to her). They said she was 1.5 years old. Recently, like a week or so ago, I noticed she had a tumor on her belly. I was worried, but was busy at the time and never got around to scheduling an appointment. The tumor is now pretty big, almost touching the ground. She still doesn't seem bothered by it, though she is a little slower than before. I am about to go on a trip (leaving on Sunday) so there is still time to schedule a surgery perhaps. However I am still not sure what to do. She could be 2 years old by now (since her birth date isn't exact) and what if She doesn't live very long after getting the surgery? since she is old, she could die of cancer or another old age disease not far from now. She is fairly healthy otherwise, but since she is older is it a good idea? Also, last time I had a vet perform a tumor surgery on my rat it cost 200 dollars and I don't know if I can afford it now (I am only 15 and have to pay for it myself). A friend suggested getting the surgery and then volunteering at the clinic to lower the cost, but do you think that would work? I knew that the vet there was experienced with rats, but I am now wondering if there is a vet that is good with rats but has lower prices? I don't know if there is time to talk to a vet and know they are good with rats as well as get surgery before we leave on our trip. I know it is all my fault that I left this off, and I hate myself for it, but now I need to figure it out because I will not leave my rat when she has a tumor and I know she will be in a horrible state when we get back. Does anyone have suggestions of what I should do? Please try to answer all the questions and thanks in advance for helpful, caring answers!