Private Sub itc_RemoveExpr(ByRef Table As DataTable)
    Dim NewTable As DataTable = Table.Copy
    Dim Row As DataRow
    Dim Col As DataColumn
    Dim AllCols As DataColumnCollection
    Table.Clear()
    For Each Col In Table.Columns
        If Col.Expression <> "" Then
            Col.Expression = ""
            Col.ReadOnly = False
        End If
    Next       
    For Each Row In NewTable.Rows
        Table.ImportRow(Row)
    Next
End Sub

if Not Convert.IsDBNull(PRow("Cost")) Then
    If Convert.IsDBNull(NewRow("Cost")) Then
        NewRow("Cost") = PRow("Cost")
     Else
         NewRow("Cost") +=  1  ' Здесь ругается
     End If
End If