some modern fans are 10W, and so they run for 100 hours on 1kWh of electricity.
I am lucky and am on a 2-year fix at 15.5p, so I could run my fan, night and day, for 4 days (4 x 24hr = 96 hours) for less than 15p
one day (24 hours) constant use would cost me less than 4pence
An hour after every shower would be far less than a penny each. Less than the cost of the shower gel.
your tariff may differ.
and the noise of a good S&P or similar is so faint I doubt you will be able to hear it outside the bathroom
more powerful fans use more, but even if it was two or three times as much, the cost of running a fan is trivial and not worth obsessing about.