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