An NFL Lockout would of course be bad, but I would be much more worried about an NBA Lockout.
NFL - 256 Regular season games
NBA - 1230 Regular season games
Nearly 5 times as many NBA games as NFL games over the course of a season, and in terms of overall turnover, an NBA lockout would cost more. Of course it depends on the location and client base of each book, but I would suggest that most books wouldnt take 5 times as much turnover on an NFL game as they do on an NBA match - therefore over the course of a season an NBA lockout would cost more...