V.W.
No offense, but if they haven't been able to find out anything yet from blood work or anything else, why do they want to do the ultrasound? What are they looking for? or is it just a stab in the dark to the tune of $1500? Has she had this dog a long time, or just a short time? How old is it?
I frankly can't afford $1500 to do an ultrasound on my dog and we've had her for years. In fact, just about a year and a half ago, we were faced with a similar situation. Our vet pretty much wrote her off, "unless we wanted to take her into the city and pay $2k for an MRI" to see if she had a tumor". Then what? Well there isn't much they could do if there is one, unless you want to invest a ton of $ in chemo. Ummm... no.
Went and got a 2nd opinion for our suffering dog. She spent the next weekend in in-patient care at the doggy hospital. They gave me a detailed expense sheet for everything they expected to do/try with her before I opted to have her treated. It was under $700... and she is back to her wonderful self again. Sure, she takes some meds daily and has regular bloodwork to check her levels nowadays... but there is no way I could have justified spending $2k for a virtually untreatable possibility just "so I'd know".
I'd have your mom check around for a 2nd or 3rd opinion somewhere else.