It costs 50 neopoints to play, but you can almost always win more than that. You can also earn Battlecards.
Keep in mind that there are only 4 of each number or letter card. So, if you have 2 fives and another player claims to play 3 fives, that player must be cheating. Similarly, if you have 4 of a kind, say Queens, and any other player says they are playing a Queen, you know they are cheating. When you know another player is cheating, accuse them.