Even though you won't be able to play the expansion for three months, you can pay for it today. That's so considerate of the studio, isn't it? Those who pre-purchase Reaper of Souls have the choice of three editions. The standard edition is $39.99 and the digital deluxe version (with extra goodies like a spectral hound minion) is $59.99. Alternatively, you can try to snag one of the collector's editions (with extra extra goodies) for $79.99 at select retail stores.
The expansion includes a new act, the Crusader character, the Loot 2.0 system, more class skills, new modes, and more.