Well, the first dog we found and who adopted us cost us nothing, apart from food, a collar and a lead- and aged 19, for the vet to come to our home to put him to sleep.
One of the dogs we've adopted since cost us quite a lot of money when the vet in France administered flea and tick treatment improperly (Practic from Novartis) which she licked on the way back to UK and caused severe ulceration of her mouth, tongue and throat- she had to be put under to examine her, as the vet thought she had a throat tumour- and then quite a long and expensive treatment. She is alive and well, but very old, about 17 (again street rescue dog, so exact age unknown)- and again, I imagine the only cost will be the final visit here by the vet.
Little dachshund we've had to adopt from DD2, also a very sad rescue case- is currently costing us a fortune for insulin and special diabetic food, and of course all the diagnosis tests. I bought him a pram as he is now blind... mind you that was only 30 Euros, and as daft as it sounds- it's really succesful- as some of you have seen from his photo in the Tatler ;) He is 17ish, and so happy to be around, I just can't bring myself to put him to sleep. I will give 300Euros to neighbour for looking after him when we go on hols. Ah well....
All this to say that people often take on animals totally unaware that if things go wrong- it can be VERY expensive. Slinky getsa regular 'cat scan' totally for free (thanks to another rescue, Pudding the cat).