> Insurers make no valuation on the land, only on the improvements
Well
$PROPERTY_VALUE = $LAND_VALUE + $IMPROVEMENTS_VALUE
Which means
$LAND_VALUE = $PROPERTY_VALUE - $IMPROVEMENTS_VALUE
$PROPERTY_VALUE is set by the market automatically, and we've already established that insurers are able to calculate $IMPROVEMENTS_VALUE, so - tada! - you've now got your $LAND_VALUE.