Conviene usare Select Case:
codice:
Private Sub Command1_Click()
Dim quote_tot As Double, perc_clienti_inc As Double, bonus As Double, n_quote As Double, quote As Double
If (num_quote.Text = "" Or num_clienti.Text = "" Or num_clienti_inc.Text = "") Then
MsgBox "Inserire tutti i valori richiesti."
Else
Select Case num_quote.Text
Case Is <= 15000
quote_tot = num_quote.Text * (9 / 100)
Case Is <= 25000
quote_tot = num_quote.Text * (18 / 100)
Case Is > 25000
quote_tot = num_quote.Text * (30 / 100)
End Select
End If
Text1.Text = Format(quote_tot, "#,##0.00")
End Sub