Valuation for insurance

Hi Everyone, I'm renewing my insurance and think it's time I upped the value of my car but am not sure what it's exact value would be. It's a 1991 Clipper Cabby in excellent condition. I've looked at similar cars for sale and their prices vary a lot.
Any suggestions or is it so variable that it's really not possible to do? 

CabrioAnna will be able to help you with this