My VB is a bit rusty, but something like this should do it:
ElseIf (e.ColumnIndex = 9) Then
Dim value As String = DataGridView1.Rows(e.RowIndex).Cells(e.ColumnIndex).Value.ToString()
Dim dTmp As Decimal
If Not Decimal.TryParse(value,dTmp) Then MessageBox.Show("Please enter a numeric value.", "Input Error", MessageBoxButtons.OK, MessageBoxIcon.Error)