Hi, if I get to Xitang from Shanghai by cab, is it feasible to walk around on our own?

Certainly, you can walk around on your own.

As I know, you can take a taxi back to Shanghai at night. The fare may be around CNY300 or 400.

Or, you may take a taxi to Jiashan South Railway Station for about CNY 80 to 100. Then, take a bullet train back to Shanghai. The last train departs at 21:21, and the trip takes about 25 minutes. A second class seat costs CNY 20.5 or 29.5; a first class seat costs CNY 25 or 48.5. :)