Dond is right, I found too many variables to even get a idea.
I talked to multiple Insurance companies and they all had different questions which related to different cost between the companies.
I just purchased a 42 ft 30 days ago and found the prices very different from 1 company to the next.
Here are some variables:
where will the boat be kept as a home port.
How much towing do you want
are you using it as your home
where will you be using it ie to and from