Honestly, my theory is that it was originally going to be free, but when they saw how popular the game was after the trailer dropped, they started working much harder, they decided it would cost money. I don't know for sure about the first part, but really, they've put so much effort into this, I feel like they would make it cost money. Plus, they don't really seem like the microtransaction type, and I don't think that would even fit with the style anyways. It's only partly multiplayer, and I don't think that locking certain gear/perks would work at all in adventure mode.