A taxi to Ping Xian costs 30RMB and a bus to Nanning 50RMB. Once in Nanning you can get a local bus for 2RMB into the city and there are plenty of ATMs/banks/moneychangers etc. So get a minimum of 100RMB per person at the border if you're heading for Nanning. Coming the other way getting to Hanoi is likely to cost you about 100,000 dong. Shared taxis run from the border to Lang Son where you can pick up a variety of buses or the train to Hanoi.