Confused and missing with this simple code, need with database code?

Above Image is my Application Snap, Im Creating Project of TV Brands Prise Listing and Invoice Generation,
This is my Code on MouseMove Events, Im trying Retrive Prise automatically from the above all combobox, with if..else condition, but there is some mistak and if i Write Qty. 2 or More then answer given is something wrong in minus(-) so how can i solve this, or else how can write this project using database can you give some Idea???

Private Sub Form1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseMove
        Dim totalwithoutdiscount As Integer
        Dim into As Integer
        Dim totalwithdiscount As Integer
        If ComboBox1.SelectedItem = "Sony" And ComboBox3.SelectedItem = "16 Feet" Then
            txtper.Text = "5000"
            totalwithoutdiscount = Val(txtper.Text) * Val(txtqty.Text)
            into = ((totalwithoutdiscount) * Val(txtdis.Text)) / 100
            totalwithdiscount = Val(txtper.Text) - into
            total.Text = totalwithdiscount
        End If
        End Sub

Please Help,,