Gahh, I feel your pain - I'm waiting for my dog to tear his cruciate (I just know it's going to happen, he's a genetic disaster, and fat and a total wingnut)
We do quite a few cruciate repairs at my clinic, using either the T.T.A (Tibial Tuberosity Advancement) or the T.P.L.O (Tibeal Plateau Leveling Osteotomy) method, depending on the case.
Yes, it's very costly and I understand many people can't afford it (I honestly wouldn't be able to without my staff discount) but it Is the dog's best option long term....Depending on the factors other people have listed above. The longer you wait the more arthritis the dog will develop which in most cases shortens the dogs life because an immobile dog is often a euthanised dog
Has your doctor given you an idea of how bad it is? Some partial tears can be "managed" with strict rest, leash walks only and medications, but the liklihood of it getting worse, or the other knee blowing is unfortunately fairly good. I hope everything turns out ok for you and if you opt for surgery and have a good vet you should be pleased.