I have done these a few times, i use a dry steam machine, fitted with a triangular brush.
Mist with a degreaser, and go over with steamer.
The steamer accelerates the degreaser, and generally removes the grime easily.
Wipe dry with a terry or microfibre cloth.
Charging is the difficult bit, try an hourly rare you would be happy with,
but charge for consumables on top, just work out how many you can do in an hour.
Andrew