For those of you that got Civ VI: Platinum Edition in the current Humble Choice Bundle offering you can get the latest expansion, New Frontier Pass for $14.99
(75% off). The price will only appear in your cart.
Once purchased you get the choice between a Steam key and an Epic key.
Edit: I'm pretty sure you need be subscribed to June's Humble Choice Bundle to get this price. Can anyone confirm?
Edit 2: People have stated that you don't need to be a have a Choice Membership to get this price. It may be that you just have to have received the offer email. Apologies for not mentioning the email. It completely slipped my mind as the focus has been on their initial (false) advertising/regional pricing. I would say that if you have gotten Civ VI PE but not gotten the discount, send an email to support and see how you go.