I'm so sorry about the knees, that's really awful when they are in pain, and surgeries are so expensive
When my boy was having back problems (we thought it was a knee at first), I used a harness for the front end, like the comfort flex harness, or the webmaster, and being able to hold some of that pressure helped some. I made one of these by cutting a duffle bag, but I wasn't strong enough to use it (max was 80 lbs): http://www.arcatapet.com/item.cfm?cat=11896
I think they have some for the back end that go around the hind legs too, but I don't have the link handy.
I hope that the vet has good news about the price!