Your poor dog! I hope she starts walking. As for the cost of an amputation, just call your vet in the morning. Vets understand that people worry about the costs of procedures, and I'm sure your vet wouldn't mind outlining the costs of the potential treatment options. Then you won't have to fret about the unknown.
Do you have anywhere you could take your dog swimming? That way she might start using her leg without having to put any pressure on it.