Last summer, 19-year-old New Yorker Ben Sann biked the city of Boston. The entrepreneur was gathering data for his website, BestParking.com, a searchable catalog of monthly and hourly parking- garage rates. Here’s what the free service can do. Say you want to leave your car near Fenway Park from 8 a.m. to 4 p.m. on a Wednesday in January. Using the site, you can easily see how to slash your parking costs in half – from $20 to $10 – simply by heading to 60 rather than 5 Kilmarnock Street.