Plastic bag bost= length*width*layers*thickness*proportion*price per kilogram.
For example: there is double layer 40cm length, 30cm width, 0.04mm thickness PP material bag, the PP material price is 14600 RMB per ton，the proportion is 0.91.
The bag cost =0.4*0.3*2*0.04*0.91*14.6=0.1275 RMB
Acctually there will be some waste in real production, we normally set the propotion as 1.
Then the bag cost=0.4*0.3*2*0.04*1*14.6=0.14 RMB
