I do clean car upholstery as part of our Car Valeting side of the business, see attached photo's.
Car upholstery does at least release stains quite nicely as you can see and I haven't had a major problem with drying after cleaning.
The downside is the fact that you could earn more by cleaning the same area of household upholstery.
The price is dictated by those lovely lads on supermarket forecourts who obviously don't have any bills to pay.
I clean using the same method as cleaning a suite, in fairness a car seat will take less time to clean so that does help with pricing.
You may need to consider cleaning the whole interior if dog smells are involved as they impregnate roof linings and odours stick to the plastic trims too where the dog has laid.
You will have to be under a £100 for a thorough clean but it might be worth £70-75.
We had the joy of cleaning 2 College Minibuses a week or so ago, never been cleaned before and more grafitti than a toilet door....at least their spelling was correct

They came up really well bt it was a really cold day and we had to employ a space heater through the door to dry off the upholstery. Where there's a will there's a way !
Les