Your rats are three, the chance that they would make it through surgery, recovery and then go on to live more than a few months is very slim.
I always say the worst part about owning rats is the short lifespan.

If you asked the vet what can be done to help the rats, then surgery will be suggested. Yes, they do want your money...I worked for a vet for a number of years and if the client wants to spend the money to try everything, they are not going to talk you out of it.
You need to say I have two elderly rats with Lipoma's and I need them to be euthanized. These are YOUR animals and YOU are paying the vet to provide a service. While their expertise is appreciated, in this case you know what you want to do and need to let them know that surgery is not an option. You do not owe them an explanation as to why. That is your business, not theirs.
Depending on the vet some will sedate with a gas anesthesia and then administer the injection or others will do an abdominal or intracardial stick. (Sorry if my terminology is off, it's been a few years). If you can find a vet that will administer the gas first, it's the best way to have it done as just the injection can be painful.

I know you love these girls dearly, but if Dexi is showing signs, then she is at the point where she can't hide the pain anymore. I think having them both done at the same time is kindest as your remaining rat won't be far behind her friend and would be heartbroken by herself.
If YOU are comfortable with your decision and you know it's best for your babies, then that's all that matters. Hugs!