how about sum() of an amount column of the selected row immediately in a textbox or label that outside the datagrid?
Hint:

1) Try and see which event is fired when you click on the row
2) If the row was not highlighted earlier then get the row number and once you get that, extract the value from the respective column
3) Add it to the textbox value...
4) If the row was highlighted earlier then get the row number and once you get that, extract the value from the respective column
5) Deduct it from the textbox value...

Give it a try. If you get stuck, post the code that you have tried and we will definitely help you