I would personally pay $200-300 for 5 fen 1981 coin. As it was stated above, official 1981 PROOF set is worth about $1000 and it contains two rare coins, 1 and 5 fens, all other PROOF coins are also worth smth about $70-100 a coin. In your set besides 5 fen 2 jiao coin has also some value. It is hard to predict for what the set will sell, but the range might be $300-500. I would try to start with $300 and see if it sells.