Me. I'd like to insure my new to me sailboat for $50,000
insurance person. How much for mast, motor, hull?
Me. I tell her and it comes to $50,000
Insurance person. How much did you pay for the sailboat?
Me. $40,000
Insurance person. That will be $X
Me. If the boat sinks and goes to the bottom of the ocean how much will I get?
Insurance person. $40,000
Me but I'm paying the premium and insuring for $50,000
Insurance person. But you paid $40,000 so we will give you that amount.
Me. If I insure for $40,000 will the premium change
Insurance person. Yes, of course
The moral of the story is you don't always get what you pay for. And I'm sure she wasn't going to tell me without me asking.

Jode5 said..
I always insure for agreed value.

Whose agreed value? I spent a year working in insurance, you and they might have had a different understanding, and they have in house lawyers.