The pilot reported that he was making his final landing of the night and had just touched down. During the landing roll as he approached the end of runway 31R, he decided to cut the corner from the runway to the taxiway and substantially damaged the underside of his right wing when the airplane collided with a sign. The pilot reported no pre-impact mechanical malfunctions of failures with the airframe or the engine that would have precluded normal operation. The damage included broken and separated lower right wing spars.