DataGridView quantity not adding up

[JL.19]

I'm currently doing a Point of Sales System but I encountered a problem, the quantity and cost of item add up just fine in the DataGridView but not in the Subtotal, Tax, and Total. I don't know what I did wrong.

Here's my code:

Private Function Cost() As Double
        Dim sum As Double = 0
        Dim i As Integer = 0

        For i = 0 To DataGridView1.Rows.Count - 1
        sum = sum + Convert.ToDouble(DataGridView1.Rows(i).Cells(2).Value)
        Next i
    Return Sum
End Function

Sub AddCost()
        Dim vat, q As Double
        vat = 3.9

        If DataGridView1.Rows.Count > 0 Then
            TxtSub.Text = FormatCurrency(Cost().ToString("0.00"))
            TxtVAT.Text = FormatCurrency(((Cost() * vat) / 100).ToString("0.00"))
            q = ((Cost() * vat) / 100)
            TxtTotal.Text = FormatCurrency(q + Cost().ToString("0.00"))
        End If
End Sub

Private Sub BtnTapsi_Click(sender As Object, e As EventArgs) Handles BtnTapsi.Click
            Dim CostOfItem As Double = 60.0
            For Each row As DataGridViewRow In DataGridView1.Rows
            If row.Cells(0).Value = "Tapsilog" Then
                row.Cells(1).Value = Double.Parse(row.Cells(1).Value) + 1
                row.Cells(2).Value = Double.Parse(row.Cells(1).Value) * CostOfItem
                Exit Sub
            End If
            Next
            DataGridView1.Rows.Add("Tapsilog", "1", CostOfItem)
            AddCost()
End Sub

And here's a sample image of what the problem looks like: