how munch does wind crystals sell on the gtn?

Impossible to say except for people who are on the same server as you and who sell them.

You might look at your server's GTN and see what they are listed for. There's no guarantee that they sell at that price, of course, but it will give you a place to start. If there aren't any listed, then list a couple at a price that might seem reasonable and see what happens. If they don't sell, your net cost is zero credits, since you get back the deposit when the listing expires. If they do sell, raise the price of the next one a bit, until you find the point at which they don't sell.