how much would a taxi from shanghai peoples square to jinshan beach be?

At least, you may need to pay around 270RMB.
It is suggested to take local subway line 1 from People Square to reach local South Railway Station and then walk to reach South Bus Station where you can find frequent buses to reach Shihua Bus Station in Jinshan District. Upon reaching Jinshan, you can take a taxi to reach the beach.
The line from the South Bus Station to Shihua Bus Station is called Shangshi-Line.