Hi. I'm afraid I can only echo what's been said.
It could be an abcess, not a tumour, but if it is, she will need antibiotics at any rate. If your rats have been fighting at all, this is very likely. Antibiotics aren't as expensive as a lumpectomy though. Being hard, it could easily be an abcess.
If it is a tumour (and sadly it could be) and it came up as quickly as you say, then she really needs to see a vet a.s.a.p because it could be malignant. I don't mean to frighten you, but a lump like that won't just go away I'm afraid. Whatever it is, your ratty needs the attention of a vet.
Could you borrow the money from someone? I have a 'visa' card that I use for my ratties' emergencies.
I really hope your little one will be ok anyway.