We may have to take the easy way out here as much as we may not want to it may be for the best.
That's a horrible decision to have to make, whether your dog is 1 or 18, and I sympathise with you. Personally, I wouldn't put down my dog, especially if I considered him/her a member of the family, (which I do, with my own pets) if it was even in part due to a few thousand $$$ of treatments. Owning pets is a big responsibility, it's a time committment and a financial committment. With respect, I feel you did as you said above, take the easy way out, as painful as that was.
Sorry you had to go through that, it's always tough.
D$