I am sorry to hear about your dog and understand how difficult it is to make decisions about health when finances are restricted. I can't give you a figure on the cost of amputation, a call to the vet you are going to use is going to be able to tell you that.

The price of surgery is going to be dependent on the type of anesthesia used, the weight and age of your dog, the length of surgery and the vet's price schedule. All of this can vary quite a bit. I can tell you from personal experience, that price shopping for surgical needs can cost more in the long run.

While not an amputation, I had a dog subluxate both front pasterns after jumping out of a second story window. The top end clinic priced the surgery at six thousand. I found another vet who could do the same for half that amount and chose him. Well, long story made short, that vet turned out to be a crook and incompetent and three surgeries and nine thousand dollars later, I still have a broken dog and the discount vet packed his practice up overnight and left the state. A little back tracking turns up that he has a long history of legal issues and since it is difficult to get any attorney to take on pet related suits, he gets away with it over and over.

So while looking at pricing, please remember to also check references and history. Don't end up in the same boat I did. Here's hoping that your dog ends up okay and you find a way to resolve the issue.

Check with vets and see if they will work out a finance plan with you
Is their a vet school nearby that might do the work on a reduced basis as a teaching opportuinty
Is there vet services on base/post that you can utilize