The ComboBox is a combination of a TextBox and a ListBox. The ComboBox displays an editing field (TextBox) combined with a ListBox allowing us to select from the list or to enter new text.
That means that you only can calculate the selected items in both your comboboxes.
Thr following code assumes integer values in columns 0 and 1 of a datagridview. It calculates column0 + column1 and puts the result in column2. Use CDbl for floating point values. To calculate the total sum just keep a running total of the values in column2
For Each row As DataGridViewRow In DataGridView1.Rows
row.Cells(2).Value = CInt(row.Cells(0).Value) + CInt(row.Cells(1).Value)