Flat line your looking about 55 - 60 mph. depending you dont change other parts. but it is possible to get them up to 100mph with the right parts.
cost wise, it depends on how much you know and want to do. i have de-restricted mopeds befor and it hasent cost me anything but as a mechanic i would charge about £100 do do it so it is alot cheaper to do some research and do it your self.