Private airport insurance?

Anyone know home much it would cost to insure a private airport? I know
some people with private fields on the sectional "No landings without prior
permission of owner" and they will tell you that it's open to all their
friends, but they don't have any insurance. I'm thinking that they are
really leaving themselves open to lawsuits if someone gets hurt due to a
rough landing (or worse). I also notice that more than a few private fields
are for sale, and I'm wondering if this is one of the reasons why.