# File lib/mail/encodings/quoted_printable.rb, line 25
      def self.cost(str)
        # These bytes probably do not need encoding
        c = str.count("\x9\xA\xD\x20-\x3C\x3E-\x7E")
        # Everything else turns into =XX where XX is a 
        # two digit hex number (taking 3 bytes)
        total = (str.bytesize - c)*3 + c
        total.to_f/str.bytesize
      end