It costs 12 pounds which is I think 15 euros. And you can do the transfer once every 3 full days.

The process itself is actually in my experience really quick. They say it can take up to an hour or more, but for me it's been less than 5 minutes each time (done it twice now). You need to be logged out of the game in order to do the transfer and in theory you need to wait for an email to confirm the transfer is complete, but for me I just logged in again once I had paid and by the time I did that it had completed already.