Well it really depends. My first ferret only cost me $25 for the doctors visit and $20 for shots once a year. Then the prices went up. Now its about $40 for a doc visit and $25+ for the shots. If you have more than one, that can get pricey, but you could always stagger their shots by a couple months, so you are not paying too much at once.
Now emergency care is pricey. When Fermi had his stroke it cost me almost $100 all together for the emergency vet to tell me he had a 15% chance of living the night, so I put him to sleep. I was in the office less than 20 minutes.
Bruno is pretty healthy, so I don't expect needing to take him to the doc anytime soon, but then Fermi was healthy too and then all of a sudden he had a stroke.
If you are lucky, doc bills will be less than $70 a year per ferret. I spend more on my own medical care in two months. I would assume anyother pet would be about the same if you took them to get shots every year.