I think bottom line how much depends on how someone can justify the expense in their daily budget without impacting financial obligations such as daily financial usage, college, retirement, etc.
I believe a major priority should be the pet's quality of life without surgery, post surgery and pain management.
A priority for us when we were deciding back surgery for one of our cocker Spaniels at age 2 was the amount of pain he would be in and his quality of life. I would not spend a huge amount of money on my pet if I knew he would be living the rest of his life in pain. We did shell out $3000 for back surgery and recovery expenses and Frankie lived to be 14.8 yrs old. Once he recovered, he was back to his old self, jumping off the wall to chase a squirrel and lived the rest of his life to the fullest.
In June of 2013 we spent $1000 on a 14yr old Cocker and he did well for 6 months and then we knew it was time to let him go. He woke up on 12/6 paralyzed and could not walk. We knew he would not survive a surgery and if he did, he had at most 2 yr to live more than likely because Cockers typically live about 15 yrs.
My 8yr old Toy Poodle cost me $6000 November 2013 and now has a bionic eye as we call it. Poodles typically live 17-20 years so he has a lot more years left if he stays safe, however, he is fearless at 10# and he charged a coyote last week with only the iron fence separating them. Ugh!
In February 2014 we elected to have hernia surgery for the 10yr old Cocker. She had a lump where her spay surgery was and we had been told it was not painful. It kept getting bigger so we had it removed when we got her teeth cleaned. You should have seen her... it was like she was thanking us when she got home. She didn't bother any stitches. Now she lays flat bellied on the cold tile and loves it. It is truly like she is thanking is for helping her feel better. That one was less than $1000.
I have family that thinks we were crazy but when we adopt a pet, we accept the responsibility and commitment to keep that pet happy and healthy as long as he/she lives. Fortunately, we are in a position that made it possible for us to care for our pets and I know some people think it is extravagant but they are my pets, my responsibility and I will not give them up to the shelter or anyone else just because they get sick or injured.
We believe in gathering all information and making an informed decision when it comes to any surgery with our pets.
I don't look at it as any dollar amount being too big. I look at it from the perspective of the diagnosis and the quality my pet will or wont have with or without surgery.
The "right" thing can be different with each family and no one is right or wrong with the decision they make based on their personal family situation.