As others have said, vet costs can add up. If your family already has a vet with whom you have a good relationship (and if that vet looks after rats), they may be willing to let you make smaller payments over time instead of having to shell it all out at once.
Depending on where you are, PetCard may also be available. It's like a credit card, but only for use at the vet. We got one about twelve years ago when one of our wee ladies needed surgery for a mammary mass. It was a risky surgery, but the vet didn't know just how badly so until Hannah was on the table. Turns out the mass was being fed by a major blood vessel. She went into cardiac arrest twice on the table; they managed to revive her the first time, but not the second. That cost around $550.
Something many of us have learned over the years, whether through our own experiences or hearing those of others, is that if you can't afford the vet care, don't take on the pet. There are often options for care if your vet is willing to work with you, but if there's no support network and the vet bills aren't doable one way or another, maybe now is not the best time to have rats.