VS 2008 Datakey - Adding Row Dynamically To Gridview

I have a situation where I needed to add a row to my gridview. So say I have rows 3, 4 and 5.

My Datakeys are defined as "ID, CostingID". I did that so I can reference the two ID values I need when saving records.

here is the code in my RowDataBound event.

Basically if it is a datarow, I'm adding the value dataitems so I can get a sub total in the footer row.

Then, I have a boolean field called isDisbursement. So I sort my data by that field. So all records with the disbursement flag as true will display first.

Then as soon as the disbursement flag hits false I add a row to display the Total of the disbursement records value field.

All this works. My problem is, because I added a row at index 4 it inherits the datakey of the row that was at index 4 before. So when I'm looping through the grid rows to save the values I'm having a problem. I've attached an image so you will understand better what I'm trying to achieve. The "Total Disbursements" is the row I added. But it gets the following rows datakey because it is now at index 4.

when I do this, as soon as I reach row 4, although it doesn't have a CostingID it gets the CostingID of row 5 that used to be row 4:

Code:
Dim CostingID As Long = Convert.ToInt64(Me.grvCostings.DataKeys(row.RowIndex).Values("CostingID").ToString)
Dim strValue As String = CType(row.FindControl("txtValue"), TextBox).Text

Code:
If e.Row.RowType = DataControlRowType.DataRow Then
If Not IsDBNull(DataBinder.Eval(e.Row.DataItem, "value")) Then
_SubTotal += Convert.ToDecimal(DataBinder.Eval(e.Row.DataItem, _
"value"))
End If

[Code] ....