There's a lot of factors for an insurance company to consider besides size. Here are some (not all) of the criteria that set the premium:
* Where will the boat be sailed?
* Age and condition of the boat? (some insurance co's require a survey before they'll cover the loss)
* Amount of liability insurance (most marina's require you to name them on the policy AND have a minimum liability requirement)
* Number of owners
* Where and how is the boot moored or docked?
* Size of the boat
* Engine size and inboard/outboard
* Loss coverage (if any). There are generally two types of coverage here (as I understand it). You can either get an "agreed value" or "actual value" coverage. In other words, you can set a value if you lose your boat, the insurance pays that. This would require a lot of discussion. So I'll leave it at that.
* Policy discounts (Discounts for licenses such a coast guard licenses, multiple policy discounts, good drivers records (yes your driving record can affect your sailboat insurance)).
For my boat, a 28 foot with the following coverage through State Farm, the premium is $168 annually.
* 300k liability
* 7k coverage for loss of the boat (probably under insured by about 5k, but still more than I paid for the boat originally... just done a lot of upgrades since then.. but whatever.. )
I have discounts for diesel and safety course (US Sailing certified).
erikdj has some great points. I would encourage you to shop around also. I just switched from State Farm to American Family, and saved 50% for similar coverage. Your mileage may vary, so shop it around.
With no disrespect, this is one of those questions for which you'd be FAR better off taking 10 minutes and making some phone calls to insurance agents rather than ask on the internet for opinions, getting the inevitable "my boat costs"..., "we switched from X to Y because", etc...
Cost of insurance is unique to your boat, it's value. your area, your insurance coverage requirements, your experience, your cruising plans, etc... all of which you need to precisely define to get a meaningful answer.
I don't how much it would cost to insure that size boat but you might want to check [EDIT]. You can get an instant quote on the website and can then compare prices with what others quoted to you. And if you like the price [EDIT]. They are also friendly and provide good service.
I don't know how boat insurance works, but I DO know how commercial insurance works. If it were commercial insurance here is what would happen with claim. Lets say you have your boat insured for $50,000, and it gets destroyed. The insurance company looks at the boat and decides that it was a damn fine boat you had there, worth $100,000. So they write you a check for $25,000 and call it a day! Why because they say you are underinsured by 50% so they only have to pay you 50% of what you are insured for! This issue is worth checking out with the insurance company! I'd read the fine print real closely.