We have only had rescue dogs and cats, and always mixed breed mongrels. Our first cat had to have his jaw wired because our charmiming neighbours hit him with a spade- and then the next cost was for the vet when he was 18.
Our Gatsby only cost us money when the French vet mis-applied practic which she then licked and it caused terrible ulceration to her mouth and throat. The vet came to put her to sleep, aged 16, in our home so she would not have to feel anxious. I sat by her and she put her head on my lap and I talked to her and stroked her, whilst the vet did what he had to do. Total cost 118Euros including taking the body away and cremation. Our previous street dog never cost us a penny apart from feeding and flea/tick treatment. until the day our vet came to our house to put him to sleep when the time came. Our current cat, Pudding, also a rescue, is about 18- and does not cost anything apart from feeding and wood pellet litter stuff.
I think pets have become so expensive because too many people have pure bred dogs which have so many health problems, and because dogs are perhaps over treated. All our animals have lived to very good long old age- but we'd always said that if one had a terrible accident or suffered from a terrible disease, we would not put them through long and painful treatment. Our DD2 little dachshund, 16, alos a rescue, is costing us quite a bit now as he is diabetic and needs a special diet and insuling- but considering the number of pets we've had over the past 44 years- it is very very little indeed.