Boston Mayor Thomas M. Menino is planning a task force to tackle an issue that has long bedeviled a city rich with nonprofit universities and hospitals: how to wring more money out of revered institutions that are exempt from property taxes. The task force will attempt to negotiate ways of increasing their payments in lieu of taxes, which the institutions voluntarily pay to varying degrees. It will examine not only the payments but also services provided by the universities and hospitals to the city and its residents, such as scholarships, internships, jobs, and other benefits.