Dim dt as new DataTable("Combowerte")
dt.Columns.Add( "Text" )
dt.Columns.Add( "Zusatzkosten", Type.GetType("System.Double"))
dt.Rows.Add("WB 2.1", 35)
....
ComboBox3.DisplayMember="Text"
comboBox3.ValueMember="Zusatzkosten"
comboBox.DataSource=dt

Dim val as double =Zylindervolumen( DmZyl.Text, HZyl.Text, TextBox3.Text, _
  TextBox2.Text)
val += CType( comboBox3.SelectedValue, Double)
TextBox6.Text = val.ToString("#,#0.00")) + 35