Interested parties can get in on the alpha for the cost of $39.99, which is the same price as the eventual final version. Larian is also selling a Duo Pack for $59.99 that includes two copies of Original Sin for co-op play, along with copies of Divine Divinity and Beyond Divinity.
According to the Early Access note from Larian, the alpha version of Original Sin currently contains the first 15 hours of the single-player/co-op role-playing game. The studio noted that "it's still rough around the edges and that certain features are missing." In addition, Larian will be updating Original Sin regularly as development proceeds, and save games will not be compatible between revisions of the game. And while the studio plans to expand Original Sin with new areas before launch, the entire game world won't be available to play until the full release.
Larian originally launched the alpha to its Kickstarter backers a month ago, after saying in September that it planned to have the full version of Original Sin ready by Feb. 28. You can check out a trailer for the Steam Early Access release above.