The Bonus Key is bought at Flinn's Shop for $5000. It unlocks the Bonus Room. It cannot be bought until after you clear the main floor. 
This Luigis Mansion Character Creator Wiki article is a stub.
Please expand it, or the Ghosts will get you! 
This Luigis Mansion Character Creator Wiki article is a stub.
Please expand it, or the Ghosts will get you! A Bonus Key.
