I have both in my kitchen. The grey salt is from Britanny and is very coarse, the fleur de sel is from Portugal and is in grains that are larger and much more irregular in form than table salt. In my household, we're not great salt users but I like to have good salt. Unlike jennyema I use both in cooking, simply because there's no point in me having yet another salt in the house. Just to give you an idea, 250 g of fleur de sel has lasted me four years, and of the kg of grey salt, I bet I've used barely 50 g.
But jennyema's right, fleur de sel is excellent for sprinkling on food just before serving. It tastes saltier than ordinary salt - like concentrate of sea breeze - and has a nice crunchy but giving texture. Can't describe it any other way.
I use the grey salt for sprinkling on fish, for example, when baking. Otherwise I grind it in a pestle and mortar.
The grey salt is different in taste. I hope I'm not being influenced by its appearance when I say it's somehow muddier in taste. If you imagine salt as musical notes, fleur de sel is high pitched, grey salt is low pitched.
I've no idea how much these salts are in the States. I bought the fleur de sel here in Spain for 8 euros. The Britanny salt was bought by my mum at the place where it's made so I have no idea what it cost. I'd say the fleur de sel is worth the price, and not just because it's lasted me so long. If you've paid a lot for a really good steak, fleur de sel makes a relatively cheap but perfect finishing touch.