Cleaning cars can be very lucrative,if done in large quantaties for car dealers and on their premises.
Stay away from private customers ,unless it is a favour for someone.In my area ,I have seen umpteen people
trying to setup valeting services for the consumer and none has survived beyond 5 years.
Pricing? Lucky to get more than £50 for an interior (standard car ,medium soil level).
By the way the HWE part of doing an interior is the easy bit ,15 min. tops.
Drying? Post towel the carpets and as said in a previous post idle the car with the heater blowing on the highest settings but keep the windows shut ,open one of the rear doors very slightly instead.