On the RCI ships that have Johnny Rockets, it is a $4.95 cover charge, but it includes the burgers and fries. On my most recent cruise I went there for lunch on both of the sea days. I think that's a pretty good deal, as if you go to Johnny Rockets on land, you can't get a burger and fries for that, or pretty much anywhere, for that matter.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)