I am so sorry about the loss of your feline friend.
Unfortunately, there is no other definative treatment for bladder stones except a surgery called cystocentesis. There are some special foods and herbal medicines that help to shrink the stones but they do not completely eliminate them.
At my practice we offer a interest-free credit card called Care Credit that can only be used for medical or veterinary services. This can be used for anyone who does not have the money for their pets care. If a owner is not approved for Care Credit I allow them to pay over time.
Until Dr. Cheng can reply, I think I can safely state that surgery is the only option in such a severe case. Sometimes symptoms don't present until the urine flow is completely blocked. As far as the "95% chance it would come back the next day," I have my doubts, but I'm sure Dr. Cheng can explain that statement.
As for the 300 pounds, it's so very difficult during an emergency to understand that practicing medicine on humans as well as animals is not entirely an altruistic pursuit. You're not only paying for the surgeon's time and expertise, but the nursing and tech salaries, clinic overhead, medicines and supplies. It's not cheap to run a medical practice of any kind. I know here in the States there are companies that offer a low or no-interest line of credit for such emergencies, and most large clinics accept it with no problem. I hope the clinic you were working with at least offered some way of working out a payment plan. If not, I suppose you can only chalk it up to a learning experience and be prepared before your next animal emergency by asking various clinics now about their payment policies, or even beginning an animal healthcare savings account on your own.
I'm so sorry you lost your friend. :-(