Games : How much network data does Minecraft normally use?

How much Internet data does Minecraft use to play "online"? (By "online", I mean not "Play Offline".) Is there a difference in usage between Single- and Multi-player? Is there anything that might affect this, such as launchers or custom skins?

Some Internet connections only allow X MB/GB per month, and I was wondering how much of that can/does get used by Minecraft, and what can affect it.