Ahead of this evening's E3 conference, a new page on the Microsoft store confirms details and pricing for the Xbox Game Pass on PC for Windows 10 players: 4 GBP / $5 USD a month.
The pass includes a library of 100+ games to dip into (although it should be noted that the library on Xbox One is almost double that) including We Happy Few, Forza Horizon 4, Hello Neighbor, Hellblade: Senua's Sacrifice, Sea of Thieves Anniversary Edition, and State of Decay 2.
Gears 5, Halo: The Master Chief Collection, and Ori and the Blind Forest are listed as "coming soon" to the PC library.