Considering snakes if cared for properly are not very likely to have issues compare to other animals you will be better off paying for the cost of treatment in case something happens.
In the last 14 years out of at least 100 adults in my care (realistically more since they come and go) I have had ZERO issue requiring veterinary intervention that means if an insurance was let's say $15 per animal per month and I had paid that, I would have spend $21000.