I am not really sure of what you are trying to do here, but if the 
datagrid:- 

1) Contains values that you have added yourself, then I don't see the 
problem. Workout the calculation yourself and add it to the grid same way 
you did with the values.

2) If the datagrid is based on a Dataset then workout your calculation in 
the SQL string (Sum, Count...etc).

3) You can work out the calculations independently, then create a table 
that holds your calculations and the Primary Key. Add a data relation to 
link both Tables in a DataSet that will be the source of the DataGrid.


Hope this helps,

Ash