Found
this info online from Hilton.
Mr. has done it once back in 2012 at Huanggong check-point. It was super easy and costs RMB 170 (supposed to be 168 but the official gave me 30 back after I handed in my 200). At the check-point there was a guy touting for visa for China too and I think it was like RMB 200 or so. Not sure about that one since we were only staying in SZ.