DataGridView quantity not adding up - vb.net

Question

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: