Just complain on the official Railway portal. I was charged five Rs extra on cold drink at Chhapra station. Since I had nothing to do with my life that day, I went to RPF to complain that day.
It also depends a lot on station and train. At some stations, they will give at MRP and at some, they will demand a markup. I remember when they used to give for twenty even at Lucknow. Now they give for fifteen only.