Here in Sweden there are now two companies which insures ferrets. It's sort of pointless to compare, as everything is different economicswise, but I know you're curious, so basically, the yearly fee is about $40, the plan doesn't cover medications and the like, nor tooth related problems. The owner gets to pay vet fees up to about $80 + 20% of the rest, up to about $800, per "treatment period" which I think is three months.
To take out an insurance, the ferret has to be between three months and three years, but nowadays there's no upper age limit to how long it's valid.
As treatment costs are rising, fees are rising too (insurance companies may not subsidise one type of insurance with the proceeds from another).