“Boardwalk Empire” is HBO’s most expensive show ever. A source close to production said the first episode of the one-hour, 1920s-based drama, directed by Martin Scorsese and starring Steve Buscemi, cost a cool $50 million. “It’s a huge period piece with a gigantic cast,” says our source. “Obviously, the rest of the episodes are cheaper because they’ll reuse props, sets and costumes, but the first one was really pricey.” A rep for HBO said, “We do not comment on bud gets.”