Unfortunately, that sounds like a typical vet bill (in my area at least). I’m pretty sure my vet charges $90 just to see the bun, even without meds, testing, or a diagnosis. 🙁
I’ve only had to give meds to Otto once, but I had good luck just backing my bun into a corner and very slowly squirting the medicine out in his mouth. They flavor a lot of pet meds, and Otto loved the Metacam and just licked it right up. I would give him a chance and see if he’ll lick it up on his own before giving it by force. I also had a gerbil on antibiotics and she loooved her meds. So you never know.
And I’m sorry your mom still doesn’t see the value of speutering 🙁 You can remind her that putting an animal down isn’t cheap either. I’ve had two gerbils put to sleep and one was $80 and the other $160! So for what it would cost to put down a bun with cancer, she might have well been spayed. Putting them to sleep isn’t free. Maybe remind her of that? My only other suggestion is to keep reminding her that your buns can’t live together and have companionship unless they are fixed. It sounds like she wants what’s best for them, so how could she want to keep them separate forever?