Nobody can tell you how much it will cost. The problem could be a $2 fuse. Or it could be the entire amplifier power supply died and that might cost anywhere from $200 to $2000 depending on what subwoofer it is. There are probably 200 brands of subwoofers with each brand having 3 to 20 models of subwoofers and subwoofers have existed since the 1950s. Repairing an older subwoofer can be even MORE expensive because it is not easy to find parts.