I checked with our current insurer (State Farm) and they don't necessarily address whether wind damage or damage to standing / running rigging and sails is covered (the policy is mute on the point). I think that means there is a good argument for it to be covered since it is an integral part of a sailing boat.
However please note that State Farm excludes weather conditions and mechanical failure as a cause for which they will cover a loss to a vessel. The way I read it: if your mast fails because of a collision, that it is covered. But if it fails because of a sudden gust while sailing in open water, I think you are on your own.
Coverage is for the cost of repair or replacement whichever is cheaper, up to the limit of the policy.
For our U20, we have $25K limit for the boat and $8K for the trailer (plus $300K in liability to tie to our umbrella policy). We pay just under $190 a year in premium.