How to measure value difference with in the same colume based on another column

Hi

I want to subrtact the values of these two freight vendors. So I want value difference for

2019-07 ~151-87=64, 

2019-08 ~ 141-88= 53

But Cost per Tonne is a measure based on the formula (Total cost/ total tonnes) on below table

Please advise how to do value difference.

Price Date                 Last name                Total Tonnes Total Cost
6/08/2019                    X1                               28.344  3819.32
6/08/2019                    X1                             28.808  3829.28
25/08/2019                 X2                             39.429  3801.17
25/08/2019                 X2                           39.673  3801.17
23/08/2019                X2                           39.696  3801.17
11/08/2019                X2                        38.565              3801.17
9/08/2019                   X2                  38.733               3801.17
8/08/2019                 X2                    37.922                   3801.17
2/08/2019                 X2                 39.2                      3801.17
2/08/2019                    X2           39.603           3801.17
4/08/2019                  X2           39.549                3801.17
18/08/2019                X2           38.755                3801.17