Oh, mock, you, Lady,and your family are in my thoughts.
My Shih Tzu, Moose, had to have both ACLs done, years apart, thankfully, and the price was right up there with what you quoted. I would think there might be a network of some kind to help post op dogs through their tough times. Perhaps a vet school would be happy to help with the hands on aspect of things. (100 lbs? Really? Oy!)
The idea of ordering through an online source is a good one. The vet's office that I worked in guaranteed to match the price of PetMeds.com, which was much less than we charged, so the client saved a good bit.
If you have any doubts about making a difficult decision, I always ask the vet, "What would you do?" And take his answer to heart. My best wishes to you and Lady.
Keep us posted on her progress.