Call me mercenary, but if you're worried about money I'd amputate the leg. Cats have a great recovery rate from rear-leg amputations and they have long, normal lifespans with no issues. One of my childhood barn cats lost a leg at age 1; she went on to have many kittens, was the best mouser of the bunch, and lived to be 15 (for an outside cat, that's a near miracle).
If you do choose the excision, I can offer hope--my parents had a dog (Lab) who shattered her pelvis and both femurs in a horrific car hit at age 2 (yes, shame on them for letting her out, but they learned their lesson). The orthopedists were able to put together the left femur and most of the pelvis, but the right femur was too badly mangled and the socket couldn't be reconstructed. The dog lived to age 9 and ran like the wind and enjoyed a tremendous quality of life with a leg that had zero ball and zero socket. When she was finally put down, it was because the old nerve damage to the pelvis had combined with normal elderly-ness and she became constantly incontinent--there was still very little arthritis and she was still happy and healthy otherwise.