Not sure whether anybody here knows, but if you have already purchased the Expansion Pass for The Witcher, you can apply
http://redeemgwent.com to purchase these decks at a discounted price. Even if you purchased the pass on XB1.
I have just received my confirmation email. It cost £14.23 including the shipping to me. It ships from San Diego, so would be cheaper for US members.
I realise it is almost as much as the PC version here, and would work out better off, ofcourse. This way would be ideal for members who don't have a local retailer stocking these and already have the expansion pass.