Sadly, it is amazing how many families put their animal down for much less severe issues. It makes me wholly sick to hear the stories from a friend of mine who volunteers at the Humane Society. If you cannot afford it, you should seek out assistance other ways. Alot of people still view a pet as a "disposable item" and not as a responsibility.
I consider myself Max and Jumars guardians, not owner.
We put Max's story at our local Pet shop ( Next to Nature ) and solicited donations to help with the TPLO surgery. We ended up recieving half the amount of the surgery and that helped us ALOT. It never hurts to ask for assistance.
In karmic exchange, I volunteer with a local metro parks supported organisation for my local park, Rogers Dog Park in Tacoma and run the park forums board Rogersdogpark.org