Think your house is a money pit? You’ve got nothing on rapper 50 Cent and his sprawling, 17-acre estate in Farmington, Conn., which has been on the market for three years with no takers. Keeping up the property costs about $450,000 a year, covering everything from taxes and insurance right down to winterizing the sprinkler system and replacing burned out exterior light bulbs, according to estimates from area real estate brokers. Add in the cost of occasional big-ticket items like roofs, furnace and painting and annual costs would easily top a half-million dollars.