Why French fleur de sel costs 230 times more than table salt

Fleur de sel is a rare, unrefined salt that is made by evaporating seawater slowly in 2-centimeter-deep ponds. It's made in western France using centuries-old methods. Just locally, it can cost 230 times more than table salt. Outside French borders, it can reach $420 per kilogram. But for salt producers in the area to make a profit, winds need to constantly blow in their favor — literally. So, what are the right conditions for fleur de sel to form? And why is it so expensive?