Minecraft will be entering Beta phase on December 20th, at which point the price will increase to 15 Euros (from 10). If you've been interested all this time but still haven't purchased, now is the time!
Oh, and this is important:
We will also change the license to remove the line that promises all future versions of the game for free. Please note that this change only affects people who buy the game after December 20, so if you got the game for during alpha, you will still get all future updates for free, despite this change. A promise is a promise.
Buying the game during beta will include all updates up until release, of course, and bug patches will be free.
So it's really important to buy now if you think you'd be interested, otherwise you may have to pay for expansions.
Notch clarified on Twitter:
Anyway, buying during beta = get everything up to and including release version for free. Expansions might cost.