We took in a rescue whose leg had been injured when she was a puppy and not repaired by her owner at the time. We did fix her leg, but hers was complicated by the fact that it had been left untreated for a year, and she grew into a very large dog (120 pounds) so her hip and knee joints became deformed. Her case involved reconstructive surgery by a specialist vet and cost us $2200. It would have cost more but we got a 10% discount for allowing her to be part of a study on pain relievers. She still walks with a limp, but her leg doesn’t hurt her at all.
A second dog was one I saw hit by a car. I stopped and picked him up – his right thigh bone was broken. That one cost $700 to fix (but he was a very sweet young dog and deserved it).
The first thing I would do is talk to your vet. Depending on what the precise injury is there may be several options for treatment, including amputation. Before you freak out, please be aware that three-legged dogs do very well. There is no stigma in the canine world to being an amputee, and the pain problem is solved. Your vet might also have suggestions on how best for you to cover the cost.
Good luck! He looks to be a real cutie.