I agree with Jennicat, you need a lot of money to own a rat, just in case. Most rats do get sick, I have a wee one here only 15 months old who has CHF (congestive heart failure), I have many here with pulmonary abscesses in their lungs and lung scarring from previous URI's (upper respiratory infections), I have 2 with PT (pituitary tumors), I have 6 with mammary tumors, I had to have my favorite pts yesterday because of a highly aggressive squamous cell carcinoma in her cheek.
They get strokes, inner ear infections, URI's, pneumonia, UTI's (urinary tract infections), females are more prone to tumours and uterine infections, males can become aggressive at 5-9 months from a hormonal aggression, which can only be taken care of by a neuter.
$600 a month, also has your own living expenses taken out of it, so its unlikely to have your $200 vet fund. Plus when pigs get sick, they get really sick. You give them meds, they stop eating, they go into stasis, they die. They are both very very tricky animals with illness. When I was with my vet yesterday she said that rabbits were the worst, guineapigs next and rats 3rd in her opinion.
You always have to have the money to pts an animal before it suffers so that is a constant amount in the Fund. So you cannot include that in the medical.
I would wait until you have a better job, and just keep researching rats if you are truly interested. They are wonderful animals, but in NO way cheap. You should see my Vet Visa bill.