Hmm, when insuring aircraft, I just tell them what I want to insure it for and they insure it for that amount.
It doesn't matter how much it's actually worth, only what I tell them I want to insure it for.
So if my plane is only worth $100K, I could just as easily get it insured for $250K...though it'll cost me that much more per year.
Sounds like the insurance company you're dealing with is too rigid in their own constructs.