we have 4 people n 4 big luggages, what is the cheapest way to travel to pudong airport in Shanghai?

Hi, SS.
Would you mind tell me your exact place in Shanghai, so that I may help you to find a reasonable method.
Generally speaking, it is suggested to take a taxi to reach the airport directly. Four of you share one taxi and the taxi fare that each other you share is not very high.

we are in gubei area near shuicheng nan lu we have 4 luggages so taking taxi is out

If so, I do not think the direct taxi will be the best choice.
You may go to Shuicheng Station of local subway line 10 to take this line to reach Hongqiao Airport, where you can airport shuttle bus line 1 to reach Pudong airport directly.
Or you can take local subway line 10 to reach People's Square Statio where you can change to take line 2 to reach the Pudong airport. But in my opinion, it is suggested to use the second method, because the subway may be so crowded.