I don’t want to tell you “do this” or “don’t do that” because while your situation sounds difficult, I’m still not in your shoes.
But I can tell you what I would do if it were my rabbit who was afflicted and I knew that he will not see a vet: I would surrender him to a no-kill shelter where he could get medical attention.
It sounds harsh, and it’s definitely sad, but I’m trying to imagine having to live every day with the awful pain of a corneal ulcer without being able to see a doctor because my grandparents told my parents not to take me.
I would want help. So I would want my rabbit to have help (competent medical treatment).
Your one experience with a vet was a bad one, agreed. It sounds like you were grossly overcharged, and I suspect that the vet was not rabbit savvy, because poopy but doesn’t usually require an automatic xray.
Could you maybe call around to any rabbit savvy vets in your area (the House Rabbit Society website has lists for the various states, provinces, etc) and see if any of them are willing to allow financing (payment in installments)? Some vets even take on pro bono (free) cases as part of their community outreach programs, where they treat the pets of low or no income people (like students).
You could also offer to do extra things around the house or help out your parents in some other way in exchange for a vet visit to a good bunny vet. Is there another family member who would be more willing to help you?
It’s also important if you do make it to a vet to not be shy – assert yourself up front. Explain your financial situation and request that any optional procedures be explained to you in advance. Ultimately, the bun should come first, but some vets like to be too thorough, forgetting that the costs could drain someone financially.
Let us know what you decide or if you have any other questions.