I'm sorry about your rats but have to say you are making the right choice, I'm honestly amazed that your vet would suggest major surgery like that on 3 year old rats at that cost. That's insane and really screams to me that money is the concern and not the animal.

When my oldest ferret who had already made it through one type of cancer was diagnosed with two more a few years later my vet was honest, she laid out the options but openly stated she would not choose to put a ferret through chemo/surgery to gain another few months with her, the suffering she would go through is not worth it and I agreed 100%. We only got a couple months with her after diagnosis but they were good months.

I got rambling there but sounds like your rats were loved and cared for, enjoy this last time with them and get some pictures taken, that's always my regret, not enough pictures to remember the good times.