My dog is at the vet right now (and for the next two day), having his surgery on his torn ACL. He's a large lab, so we opted for one of the higher level treatments (sorry I can't be more specific, they explained three different things and I almost passed out hearing the specifics of what they do, so I can't really explain). His surgery, including the x-rays and bloodwork is estimated at 2,200.
One thing I might recommend is looking into Pet Insurance ASAP!! Most of what we saw won't cover preexisting conditions, but they may honor it if you get the insurance before going to the vet. We are getting it for our dog now, and while it won't work for this surgery, the vet said that of all dogs that tear one ACL, there is a 40% chance they will tear the other one.
I hope your dog feels better; my guy has been so miserable because he can't play like he is used to...hopefully the recovery time is really only as long as they say it is (2 weeks of very restricted movement using a sling to help him walk (which will be fun with a 90 lb dog!) and then a month of restricted play, like no running and jumping).
ETA: You may want to start a joint supplement such as Cosequin. I know it won't help with the ligaments, but the bones can start to grate together and arthritis can form. Our new vet recommended it, but since we have a large dog in a breed prone to hip problems, he's been on it preventively for the past 3 years anyways.
Oh, and don't let the fact that your dog is walking on his leg make you think it's getting better. We thought the same thing (he was even lifting the other leg and standing on the bad one to go to the bathroom). But when the vet moved it around, after waiting the time period (3 weeks) to see if it was anything else and would heal, it still moved wrong. It's unfortunate, but they told us that dogs, and some breeds in particular, are very resilient and won't let pain really get them down...they still want to play and be puppies, so they will just learn to deal with the pain.