How much would it cost to fix the overdrive in a 1990 Mitsubishi 3000gt?

Someone is selling a 1990 Mitsubishi 3000gt pretty cheap but he said the overdrive is broke or something, didn't say exactly what is wrong with it. Was just wondering how much they usually cost to fix?|||most likely the transmission is shot and would cost more to rebuild than the car unless its in mint condition with low miles or get a used working one and do it your self your only cost would be the transmission at a shop prlly 2500 or more