The basic Sigil add-on is free beyond the price of the game itself. If you want a soundtrack from metal guitarist Buckethead, though, you'll need to pay €6.66 (about $7.46). The limited-edition physical copies have long since sold out, unfortunately -- if you want something tangible, you'll have to make do with a t-shirt. If you can live with that, though, this is about as close as you'll get to a third Doom game using the original engine.