I'm totally against the option "Being able to buy cards with real-life money". In my hopinion a good game is a game where the strongest achieved their result only for in-game abilities, not because they've payed real money to have unfair advantages.
I believe that a good way to receive money for developers is to create a supporter status: supporters don't have real in-game advantages, but they have some useful options. For example, the saving deck option is something that is perfect to be put in supporter pack: it makes life easier for supporters, but you don't need to have it to reach the top. Other things to be added in a supporter pack could be a list of friends, some shortcut buttons and so on.