if the battery is relatively new and it's dead to the point where you gotta replace it... replacement is free under warranty.
if you keep forgetting to turn lights off, then see a professional to install relay chip that will automatically shut off your light when your engine is off. It's really a small price to pay compare to buying new battery, towing truck, and labor fee all together.
My brother's old Infiniti automatically shuts off lights when engine is off and when he exits his vehicle. most modern cars too but I don't understand why my Xterra doesn't 
but it automatically shuts off after a few minutes.
