Over the years, Charlotte leaders have helped build a 700-room hotel and schmoozed Washington insiders to convince Republicans and Democrats to stage a national political convention here.But as the parties prepare to solicit bids for their 2012 conventions, Charlotte officials are asking a different question this year: Is it worth it?A convention could pour millions of dollars into the local economy and bring a raft of free publicity from media around the globe.But Mayor Pat McCrory says hosting a convention could cost $60 million.And experts say the economic benefits and media exposure are often far smaller than city boosters tell the public.