1997 Renault Clio Automatic Transmission Problem

Hi, I own a 1997 1.4 auto clio. It wont select gear and seems to be constantly in neutral even when I moved the shift into D or R. I have checked the transmission fluid and topped up in order to check a response and got a little bite when D or R selected but required high revs in order to enerate power to move. I have noticed red transmission fluid on a component looking under the vehicle and considered a possible leak. Was wondering if this sounded like a gear box issue or transmission fluid/reservoir issue? Would it be costly to repair?