how to get total of a filed value in table merger

[Markisio]

hi there

Would like to get total of a field in a table merger, here is my example of table meagre

Product - Product Cost - %OfCost

Apple - 10 - 12.5%

Orange - 20 - 25%

Lemon - 50 - 62.5%

I am hoping I can do something like this in %OfCost: ([Product Cost]/[sum_cost])*100. lets take an apple for example: (10/80)*100. 

10 is the total cost for apple

80 is the total cost for all products 

in my table I have the following

Product - Cost

Apple - 5

Apple - 5

Orange - 4

Orange - 4

Orange - 4

Orange - 4

Orange - 4

Lemon - 10

Lemon - 10

Lemon - 10

Lemon - 10

Lemon - 10

Hopefully there is a solution for that :) thanks for all your help in advance