When Kevin Spacey brings his production of A Moon for the Misbegotten from London’s Old Vic to Broadway this spring, it could quickly become the hottest show in town. And it had better, because Spacey is apparently demanding a salary of $25,000 a week plus a hefty chunk of the box office take for his services, which is causing some nervous investors to wonder whether, on a limited 10-week run, they have any chance of seeing any profit.