Um... pretty sure the devs have said a few times that you aren't going to be paying monthly for this game; you would buy a passport once to gain access to the game, and there are micro-transactions, as someone has already mentioned in this thread, where you can purchase cosmetics and the like; nothing like guns though.
Unless they've changed their previous statement, if there are any expansion DLCs in the future that unlock more areas and such, you'll have to purchase a passport to access that content.
TLDR
Other than that though, the game is technically free-to-play, a lot like guild wars where you pay once and that's it minus DLC. If i'm wrong, please do correct me..