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The comics cost $2 each; the 71 currently available titles include the Astonishing X-Men, Captain America, Marvel Zombies, X-23 and X-Men Age of Apocalypse.
Unfortunately, as is so often the case due to copyright restrictions, all of these are only available in the US. The folks at Comixology wrote only a short post about the addition to their catalog, which also includes comics by publishers such as Arcana, Asylum Press, Evil Twin Comics, Red 5 and Studio 407, so it's hard to say when (if ever) the US-only limit will be lifted and whether any additional Marvel titles are coming.