Standard insurance carriers require homes to be insured to 100% of the replacement cost. The market value of your home or land is not considered in the insurance contract. Homes built prior to 1943 will be much more expensive to rebuild, as the materials used pre-1943 are much different than materials used post 1943.
To illustrate: A 4000 sq. ft. home located on beach front property in Miami may be valued at close to a million dollars on the market, while that same home located in Cornfield, Nebraska might be worth $250,000 on the open market. If both of the homes burned down, the cost to re-build either, assuming the same materials are used, would be very similar. This illustrates the difference of replacement cost versus market value and why the insurance carrier is more focused on the amount it costs to rebuild a home than what it can be bought and sold for.
