Blizzard has detailed what comes with the collector's edition of Diablo III: Reaper of Souls. On top of the game you'll get a behind the scenes DVD and Blu-Ray two-disc set, soundtrack, artbook, and mouse pad. As for digital content, you'll get a spectral hound minion, aesthetic artifacts, World of Warcraft treasure goblin pet, and some portraits related to StarCraft II.
The Diablo III: Reaper of Souls collector's edition will run you $79.99 at select retailers. There's also a Deluxe version for $59.99 that contains a bunch of extra digital content too.