Notch has made the following tweets :
Minecraft’s model included only a one off payment, so it is likely 0x10c will to as opposed to the earlier speculated monthly subscription model. The earlier you purchase the game the cheaper it will cost so get your copy for pre-alpha testing as soon as it is available! Minecraft initially costed €9.95 (~ $13/~£8) during the alpha phase, so we could be expecting similar pricing, or something completely different!
Notch has made another tweet:
@GamesC3 single player won’t have any subscription fees.
No mention about online multiplayer, so we could still be looking at a subscription fee there.