It is an unfortunate fact of life that rats are an expensive pet to keep, even if you obtained all your rats for free like I did. It's entirely likely that you will spend more on a rat in their three years of life than you will on a dog in their twelve years of life. In fact, my dog has never gotten sick and is on a Limited Vaccine Protocol, so I'm willing to bet if I did the numbers, my rats already outweigh him in vet bills. If I end up having to get Whiskers neutered, they definitely will.
Any pet has the potential to become very expensive, though, so the best I can recommend to someone who cannot afford a vet is to re-home their pets to someone who can. Unfortunately, in most places, there isn't a law requiring adequate, timely medical care for pets as there is for children.