Many bank customers have long felt nickeled-and-dimed by fees imposed by financial institutions. But now some banks are demanding much bigger denominations. The region’s largest banks are socking customers with charges of as much as $50 a month for checking or savings accounts if they don’t meet certain requirements. Citizens Bank charges customers $50 a month when the balance in a money market account slips below $1,000. “I haven’t seen anything like $50 a month before,’’ said Greg McBride, at Bankrate.com.