Paul's code may work for you. (p.s. I guess not, as Paul assumed as I did that you were subtracting dates, not trying to subtract DateTimePickers).
My code would need to be updated to use the Value of the DateTimePicker.
Code:
TotalPrice.Text = ("£" & 9 * People.Value * (DepartureDate.Value.Subtract(ArrivalDate.Value).Days)