Minecraft's world building has proven such a hit with my kids that I bought a second license with the intention of setting up our own private server for a bit of multi-player world building. Both my licenses are alpha licenses which cost €10 each. Sometime today the beta starts and the price goes up to €15.
If you have any interest in Minecraft but haven't gotten around to buying it yet I strongly recommend buying before the price goes up. It isn't just a matter of the extra €5. Alpha purchasers also get "all future versions of the game for free" while beta purchases will only up to the first release free. Given the success of the game this makes the alpha purchase an incredible bargain for the few hours that remain at that price.