Amazon has half off on the Blu-Ray+Digital of Andy Weir's super-sciencey sci-fi/maybe someday reality novel turned-film currently.
It's even cheaper than the DVD, which is on sale for $13.99.
It's one of the few films I've seen that stayed very true to the source book--although they had to cut out a lot of Watney's MacGyvering; it would have turned into a 4 hour movie the length at which it goes on in the book.
It also has 10 extra minutes incorporated, a few extra scenes on the second disc, four more Ares Mission videos, and hours of making-of with NASA contributions.
Good choice for science buffs or gear heads. Great story too.