The costs mainly depends on your travel itinerary in Shanghai. It is generally expensive. You may refer to Shanghai Tours(https://www.travelchinaguide.com/package/shanghai.htm) .It includes the routes, attractions and price of travelling in Shanghai.
Well, I think 12 days are too long to visit Shanghai, 4-5 days are enough, you can use the left days to have a side trip to other cities near Shanghai. See Memorable China Tours(https://www.travelchinaguide.com/tour/memorable/) provided on thie web, which are really worth visiting.
Anyway, it is up to you, wish you a wonderful time in China.