Miss Piccalilli needs surgery

Miss Piccalilli was rescued by my daughter and I. She had been at the ATM for days, walking back and forth, assumption is she was abandoned and didn't know the area. As I wasn't looking for a dog we took her to the SPCA. They didn't want her and advised Animal Control but said they might put her down. We took her to the local vet and they advised she was healthy. She got microchipped, spayed, flea treatment etc and then registered (That was an expensive month). Within a few days she was limping so I took her back to the vet and found she has a luxating patella. They weren't sure how as it isn't standard for her breed but it was suggested it was due to unknown trauma (human or car). She has had 3 rounds of pain killers/anti inflam meds this year alone and it is a constant difficulty for her now. The operation to repair the patella is $3500. As a working single mum my finances are tight but I can save $50p/fn towards her operation and would continue to try to get there myself but it will take a long time and she is sore all day every day now. I can't keep giving her pain meds as they mess with her stomach. I've changed her food to the expensive joint care one and I massage her regularly but she can't even jump in the car or on my bed anymore. She needs help please. She is my best friend