Every month I spent about $40 on food for four ferrets. They were fed a raw diet. A kibble diet will cost far more.
Litter, you can use pine stove pellets. A 40lb bag is about $5, that lasts awhile.
Toys, however much you want.
Treats, go for raw meat. Mine adored pork.
Beds and hammocks, again, however much you want. You can even make your own.
Vet care, I would say almost equal to a dog or cat. Although you can get a titers to see if you even need to vaccinate, if not you save money and they are still protected. Although if its required by law or you let strangers around your ferrets, I reccomend a rabies vax every year. Its pretty cheap though.