How do I get from Shanghai to Zhouzhuang?

No train from Shanghai to Zhouzhuang. You need to take direct bus. Then 40-60 minutes from Kunshan Bus Station to Zhouzhuang. It takes 1.5 hours from Shanghai South Coach Station to Zhouzhuang directly. Easier to take direct bus from Shanghai.

Will cost a lot to take a taxi as you will need to pay for return trip (I was told will cost over 400rmb)

The bus from Shanghai to ZhouZhuang is reasonable comfortable even for a travel sickness sufferer like me. ZhouZhuang is actually quite close to Shanghai, it is mostly slow city drive in heavy traffic. Only about 20min on rural road then over 1 hour on city road of Shanghai region. So taxi won't go must faster.
No train to zhouzhuang

Yes, you can take a train to Kunsan then a taxi to zhouzhuan, but that will take even longer and lot more expensive.
Train to kunsan might take 45min (waiting, getting on and off etc) , then 1.5 hour taxi. Direct bus is only 1.5 hour and cost about 40rmb.