I mean, if your house costs a million and gets destroyed by fire every year I can insure you for 100k per month, so it is insurable just. Not usefully.
I would argue that if the cost of the insurance exceeds the available money, that counts as de facto "uninsurable".
Tautologically false: when "you can't afford it" because there isn't enough money to pay for it, you cannot in fact buy it.