it may sound harsh, but you have to forget about the money if your cat is still unwell. if her back legs are not working, you have to take her to a vet. it may be that you just go to the pdsa and show them the state of your cat and beg/plead/cry and get her treated. you have to do what is in the cats best interest, not yours. that is the responsibility of being a pet owner, i'm afraid. i would be in the same boat as you if one of my 3 cats became seriously unwell, but i would not leave any of them in pain, distressed or clearly unwell. i still think you should be taking her to the vet.