I've seen cats who had broken/paralyzed hind quarters get along perfectly well in the wild. I'd try to figure out if it IS a re-injury. As a house cat, he'd be in much less danger of reinjury as he would be in the wild.
I suppose the major important question is; is he in continual pain from this injury? If not, or it's manageable, then you can put off having the surgery until the situation changes.
Whoa-that is pretty expensive, but many vets will take payments. I know I couldn't come up with that kind of money all at once. I don't know if I'd choose to amputate or not, that would depend if the cat was in pain or not AND whether the bad leg was preventing him from getting around easily. If he could get around better without the leg, and the expense won't put you in a bind, then I'd do it. I have seen sooo many animals do fine with 3 legs. In fact, they probably handle it better than we would. Animals are true survivors.
Good luck