So sorry you're going through this. It's never an easy decision.
My best advice is to take the information the vets give you, keep Blanca's best interests in mind and make the best decision you can. You can only do your best--and as long as your decision is made out of love for your girl, it will be the right decision.
The hardest part of pet ownership is knowing when it's time to let them go. It never
feels right and I always have to fight the urge to second-guess myself after the fact. But because we live in an imperfect world, we can never know
when the perfect time is. Best we can do is make an educated guess...
How long will it be before you get the results of the ultrasound? With luck, you'll have a better idea of what's going on after you get the results. Might make it a bit easier to make a decision. If more tests are suggested, look at them carefully to see what the results might tell you. If the results won't change the outcome or the treatment (or lack thereof), the test is probably just an unnecessary expense. If you can't afford it, you can't afford it, plain and simple. No guilt.
Regardless of how this turns out, please post back and let us know how it goes. Most of us have been where you are at one time or another and have an inkling of what you're going through.
Hang in there and remember to breathe!