A fiscal watchdog group is questioning the state’s century-old prevailing wage law for construction workers, saying it unnecessarily costs taxpayers billions of dollars a year in added expenses for big road, bridge and other projects. The Empire Center, a fiscally conservative budget watchdog group, looked at the state’s constitutionally protected prevailing wage law. It requires contractors on public projects to pay their workers the amounts set in unions’ collective bargaining agreements. The Empire Center’s E.J. McMahon said an analysis of federal data on wages paid finds the law’s interpretation is outdated and that New York may be paying more in taxpayer money than is necessary — up to 25 percent more for some projects in some regions of the state. “You’re talking about something that’s neither prevailing nor limited to the wage,” McMahon said.