Land value when no tax assessment exists

[Stephen Race]

Hi, I purchased a rental property in a beach front condo complex on Hilton Head Island and am trying to determine the land value. Talking with the Town, they do not assess the land and building separately (like most taxing authorities do). They did not know of a good way to value the land. Speaking with 2 realtors, there is no comparable size/location land only property for sale or sold recently. I talked to the management company and got the insured value (only building is insured), but that is only about 30% of the total assessed value, because the building insurance just rebuilds to original shell. I am at a loss to find a way to determine the land value. Any experience on what to do in these cases?