A 54:

Your BAB does not change, and is a single number. Many other modifiers can affect your attack bonus, up or down, but your BAB is unchanged. If you're a level 6 fighter with a 10 strength and nonmagical weapon, and no other modifiers, and you're prone, then your base attack bonus is +6, your total attack bonus on your first attack is +2, and you can make a second attack in a full attack with a total attack modifier of -3.