It's often part of a sale, and especially cheap when bundled with the first Bioshock (which you should play ASAP if you haven't yet). On GMG it goes for a fiver right now, which is where I bought the DLC from today too, so that's a good price if you want it right now.
I've seen good bundles with Infinite included too, so if you haven't play the third one too I'd just wait for one of those packs to reach ten bucks.
Ya, I've already played the first one, but I skipped second, might as well buy that steam bundle at the next sale. I have BIoInf though, so I need first and second bundled, I remember I've seen such a bundle on steam.
edited: so, I bought that indie gala bundle, but it seems I have all games but one... I really need to make some list, I seem to have no clue what I have outside 200 or so games I remember I have. (><)