Vb + sum decimal textbox

Private Sub TextBox8_TextChanged(sender As Object, e As EventArgs) Handles TextBox8.TextChanged
   ' Calcular Precio Final Con IVA
   Dim Sin_IVA, IVA, Total_IVA As Decimal
   If Not Decimal.TryParse(TextBox8.Text, Sin_IVA) Then
      '' Handle here the case where Sin_IVA is not a proper decimal value
   End
   If Not Decimal.TryParse(TextBox9.Text, IVA) Then
      '' Handle here the case where IVA is not a proper decimal value
   End
   '' If you get here, both values have been validated, and you do not need to repeat the parsing process.
   Total_IVA = Sin_IVA * (1D + IVA / 100D)
   Dim stringRepresentation As String = Total_IVA.ToString("#0.0000")
End Sub

Imports MySql.Data.MySqlClient
Imports System.Configuration
Imports System.Resources
Imports System.Decimal
Public Class CosteMaquinaria
    Dim MysqlConn As MySqlConnection
    Dim COMMAND As MySqlCommand
    Dim READER As MySqlDataReader
    Dim a As New OpenFileDialog
    Dim dr As MySqlDataReader
    Private Sub CosteMaquinaria_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        TextBox9.Text = "21,00"

        MysqlConn = New MySqlConnection(ConfigurationManager.ConnectionStrings("test").ToString)
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        MysqlConn.Close()
        MysqlConn.Open()

        MysqlConn = New MySqlConnection(ConfigurationManager.ConnectionStrings("test").ToString)

        Me.Cursor = Cursors.WaitCursor
        Dim SDA As New MySqlDataAdapter
        Dim dbDataSet As New DataTable
        Dim bSource As New BindingSource
        Try
            MysqlConn.Close()
            MysqlConn.Open()
            Dim Query As String
            Query = "select Numero_Interno,Matricula,Marca,Numero_Serie,Modelo,Tipo_Maquinaria from xCantera.Flota"
            COMMAND = New MySqlCommand(Query, MysqlConn)
            SDA.SelectCommand = COMMAND
            SDA.Fill(dbDataSet)
            bSource.DataSource = dbDataSet
            DataGridView1.DataSource = bSource
            DataGridView1.DataSource = bSource
            DataGridView1.Columns("Tipo_Maquinaria").Visible = False
            SDA.Update(dbDataSet)
        Catch ex As Exception
            MessageBox.Show(ex.Message)
        Finally
            MysqlConn.Dispose()
        End Try
        Me.Cursor = Cursors.Default
    End Sub

    Private Sub DataGridView1_CellClick(sender As Object, e As DataGridViewCellEventArgs) Handles DataGridView1.CellClick
        If e.RowIndex >= 0 Then
            Dim row As DataGridViewRow
            row = Me.DataGridView1.Rows(e.RowIndex)
            TextBox1.Text = row.Cells("Numero_Interno").Value.ToString
            TextBox2.Text = row.Cells("Matricula").Value.ToString
            TextBox3.Text = row.Cells("Marca").Value.ToString
            TextBox4.Text = row.Cells("Numero_Serie").Value.ToString
            TextBox5.Text = row.Cells("Modelo").Value.ToString
            TextBox6.Text = row.Cells("Tipo_Maquinaria").Value.ToString
        End If
        'Seleccion Combobox1
        Try
            MysqlConn.Open()
            Dim Query As String
            Query = "select Categorias from xCantera.CostesMaquinaria_SubGrupos where Tipo_Maquinaria = '" & TextBox6.Text & "' ORDER BY Categorias ASC"
            COMMAND = New MySqlCommand(Query, MysqlConn)
            READER = COMMAND.ExecuteReader

            'Reinicio Combo
            ComboBox1.Items.Clear()
            'TextBox1.Clear()

            While READER.Read
                ComboBox1.Items.Add(READER.GetString("Categorias"))
            End While

        Catch ex As Exception
            MessageBox.Show(ex.Message)
        Finally
            MysqlConn.Dispose()
        End Try
        'Seleccion Combobox1

        If ComboBox1.Items.Count > 0 Then
            ComboBox1.SelectedIndex = 0    ' The first item has index 0 '
        End If

    End Sub

    Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged

    End Sub

    Private Sub TextBox6_TextChanged(sender As Object, e As EventArgs) Handles TextBox6.TextChanged

    End Sub

    Private Sub TextBox10_TextChanged(sender As Object, e As EventArgs) Handles TextBox10.TextChanged

    End Sub

    Private Sub TextBox8_TextChanged(sender As Object, e As EventArgs) Handles TextBox8.TextChanged
        ' Calcular Precio Final Con IVA
        Dim Sin_IVA, IVA, Total_IVA As Decimal

        Sin_IVA = TextBox8.Text
        IVA = TextBox9.Text

        Total_IVA = ((Sin_IVA * IVA) / 100) + Sin_IVA
        TextBox10.Text = Total_IVA.ToString

        TextBox11.Text = Total_IVA - Sin_IVA

    End Sub

    Private Sub TextBox9_TextChanged(sender As Object, e As EventArgs) Handles TextBox9.TextChanged
        '' Calcular Precio Final Con IVA
        'Dim Sin_IVA, IVA, Total_IVA As Decimal
        'Sin_IVA = Val(TextBox8.Text)
        'IVA = Val(TextBox9.Text)

        'Total_IVA = ((Val(Sin_IVA) * Val(IVA)) / 100) + Val(Sin_IVA)
        'TextBox10.Text = Total_IVA.ToString
        '' Calcular Precio Final Con IVA

        '' Calcular dif Precio Final Con IVA
        'TextBox11.Text = Val(Total_IVA) - Val(Sin_IVA)
        '' Calcular dif Precio Final Con IVA
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        'Insertar en Base de Datos
        Me.Cursor = Cursors.WaitCursor
        'Dim MysqlConn As MySqlConnection
        Dim Sin_IVA, Con_IVA, Diff_IVA As Decimal
        Sin_IVA = Val(TextBox8.Text)
        Con_IVA = Val(TextBox10.Text)
        Diff_IVA = Val(TextBox11.Text)
        Dim COMMAND As New MySqlCommand
        Dim READER As MySqlDataReader
        Dim a As New OpenFileDialog

        Try
            MysqlConn.Open()
            COMMAND.Connection = MysqlConn
            COMMAND.CommandText = "INSERT INTO  CostesMaquinaria (Numero_Interno, Grupo_Gastos, Numero_Factura, Precio_Sin_Iva, IVA, Precio_Con_Iva, Diff_IVA) 
            Values(@Value1,@Value2,@Value3,@Value4,@Value5,@Value6,@Value7)"
            COMMAND.Parameters.AddWithValue("@Value1", TextBox1.Text)
            COMMAND.Parameters.AddWithValue("@Value2", ComboBox1.Text)
            COMMAND.Parameters.AddWithValue("@Value3", TextBox7.Text)
            COMMAND.Parameters.AddWithValue("@Value4", TextBox8.Text)
            COMMAND.Parameters.AddWithValue("@Value5", TextBox9.Text)
            COMMAND.Parameters.AddWithValue("@Value6", TextBox10.Text)
            COMMAND.Parameters.AddWithValue("@Value7", TextBox11.Text)
            READER = COMMAND.ExecuteReader
            MsgBox("Data Save !", MsgBoxStyle.Information)
            MysqlConn.Close()
        Catch ex As Exception
            MessageBox.Show(ex.Message)
        Finally
            MysqlConn.Dispose()
        End Try
        Me.Cursor = Cursors.Default

    End Sub
End Class

CREATE TABLE IF NOT EXISTS `CostesMaquinaria` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `Numero_Interno` varchar(255) DEFAULT NULL,
  `Grupo_Gastos` varchar(255) DEFAULT NULL,
  `Numero_Factura` varchar(255) DEFAULT NULL,
  `Precio_Sin_Iva` decimal(10,3) DEFAULT NULL,
  `IVA` decimal(10,3) DEFAULT NULL,
  `Precio_Con_Iva` decimal(10,3) DEFAULT NULL,
  `Diff_IVA` decimal(10,3) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=11 DEFAULT CHARSET=latin1;