The only time I had to put a pet down was when my husky/shepard Shennlly- how do you spell that, she was named after my favorite rye, was ripped open by a black bear.
I came home from work and instead of seeing her lovely white fur she was instead stained red and pink with blood, and her liver was hanging out from a vicious wound to her under belly.
There was no way to treat this injury, so I go out the 303 and shot her in the head. She just sat there almost dead before I made the shot, I don't think there was much blood left in her.
In this case I had to make this choice, but otherwise I have let my pets decide when they want to pass on. Not long ago my 15 year old cat began to lose weight. He had kidney problems, no pain but just that old age decline. He remained happy to the very end. I came home from work, and he climbed up the stairs from the basement to rub himself around my legs, and share a moment- his last moment with me on my lap.
At this point he was skin and bones. He purred, licked my hand and cuddled. Then he nearly fell off my lap when he wanted to get down and use the litter box. At this point I knew he would not walk back up the steps. The next morning he was dead by the door hugging my boots. In retrospect it was a very beautiful death, as he wanted to hold onto me and his posture was like he was at rest. Satiba was a great cat.
The cost of a pet is huge. Its not the food or vet bills. The huge cost is the fact that we out live most pets lifespans. It is a difficult time when a pet must pass on- that is the biggest cost of all.