я предположил, что количество Cell 0
, цена Cell 1
и сумма Cell 2
For i As Integer = 0 To DataGridView1.RowCount - 1
Dim xQty as Double = CDBL(DataGridView1.Rows(i).Cells(0).Value)
Dim xPrc as Double = CDBL(DataGridView1.Rows(i).Cells(1).Value)
DataGridView1.Rows(i).Cells(2).Value = xQty * xPrc
Next