I did a number of them years back, while I still had knees that worked.
I put it on a time basis, at that time my wtw cleaning was $60.00 per hour, so that's what I charged for car interiors.
This includes carpets, doors and overhead.
Took an average of an hour.
I did prefer doing RV's though, not as much knee work on gravel driveways.
A couple of the right tools helped, too. Such as an extended nose detailing tool, Rug Rat turbine vac, etc.
Gary