There is no legendary artifact at the end of the current museum sale, all you get is a bunch of crappy 2/3/4 star random artifacts and lots of fragments, the last unlocked sale is $39.99 for 10 rare artifact chests with a chance of getting a 2 star (72%), 3 star (24%) or 4 star artifact (4%). I think it was 4 levels deep to get to that.