I wish that it was easy, but it isn't. Our power catamaran is 7 years old, we are limited to coastal cruising in the US and Bahamas, and last year paid a bit under 1% of insured value. Our 20 y.o. sailboat, blue water coverage, we paid about 2.5% of insured value, and it had to be surveyed every 5 years.
However, I can't say that a boat worth more than ours would have a similar premium ratio. There are just so many variables that only an insurance professional can give you credible information. I've heard many complaints from boat owners that their insurance is twice their neighbor's - that's a meaningless comparison, as you can imagine. Getting competitive quotes is the best approach. A reputable broker will explain the differences in coverage and explain everything to you.
Now I'll tell you a story of a foolish mistake by us that could have been a terrible problem for us.
Back in the 80s we spent four or five months of the year in St. Martin in the Caribbean, and would head for Venezuela for hurricane season. One year a friend of ours introduced us to a friend of his, an insurance agent in the US. The agent convinced us that he could get us cheaper insurance, same coverage. So we gave him a copy of our binder, and he advised us that he had, indeed, found better insurance for us. We emphasized that our coverage had to be the Caribbean down to Venezuela, and he assured us that that was indeed what our coverage was. We sent our payment, but did not receive a binder from him/his insurance company. We asked several times, and didn't hear from his, other than, "it's in the works, don't worry." Well, circumstances kept us in St. Martin into hurricane season, and in September of 1989, Hurricane HUGO came through. For much of the Caribbean, HUGO was a horrible hurricane. St. Martin did not receive a direct hit, though the hurricane stalled offshore so we had more than 24 hours of hurricane force winds. Watermelon came through fine, but a lot of boats didn't. Shortly after that, we finally received the insurance binder. It covered Watermelon to 50 miles offshore OF THE US VIRGIN ISLANDS! NOT St. Martin!
We were furious, and when we called the broker, telling him that the insurance was useless, and what would have happened if our boat had been damaged by the hurricane, his response was "don't worry, the insurance company won't know where St. Martin is, they'll think it's the Virgin Islands." It was an expensive mistake on our part because we had to then scramble to get appropriate insurance.
I have never heard of anyone else having this problem, so it might be an unnecessary warning, but not every insurance agent out there is competent and honorable. Be sure you do your homework and choose a well-rated company and a broker with references, and not from a friend of a friend.