Assassin’s Creed Unity will have an alternate in-game currency which you can buy with real-world money.
Along with the standard currency used by the characters, you can buy “Helix Credits”, presumably with your credit card. Helix Credits will allow you to access boosts or unlock certain customizations (that you’d otherwise have to save up for with your ingame money), or reveal hidden stuff on the map without having to go to a viewpoint. So, for at least some features, you’re paying for convenience.
However, you can also buy boosts which enhance your abilities for a short while or upgrade existing weapons by “hacking” them, for considerable benefits: