YGO!O is pay to play. Rather than monthly fees, it's got a system where you buy Duel Points, and then expend them for duels. Mean they can appeal to a wider range of players, and parents can have some degree of control to how much time thier children play the game.
Duel Points - Nessisary to duel, must be purchased, via the Duel Pass System.
Duel Pass - Obtainable through offline retailers, not sure who all's carrying them, or via the YGO!0 Website. From the wbsite it's $14.99 for 150 Duel points. Not that bad, works out to 5 games a day, or there abouts. Roughly $0.10 a duel. Not sure on the offline price. 1 free card per 30 points it looks like. Website cards seem to be randomly chosen.
And incidently, the word filter in game is one of the poorest I've ever seen.