The base version of the mobile game is free.
The base version of the Steam game costs money.
I have purchased content on the mobile version (iOS), and after I purchased the Steam version and linked my accounts, all content that was on mobile was available to me, as such there is some content that I have only paid for once.
The content that I have purchased on both Mobile and Steam is Adventure 3, 4, 5 and 6, as I purchased these on mobile and they are part of the base game on Steam.
As an early adopter and supporter I certainly have been screwed by Obsidian's decision, but if I were a new user, then paying $25 for the base game (without discount), or $40 for all content (without discount) and being able to access all the content on Steam and mobile platform sounds like a pretty good deal.