Your car looks immaculate in your avatar, like it doesn't need a respray, your colour does "pop" out more or should I say my colour doesn't "pop' as much as a black or darker colours and off course we can't see the little details or faults only we know are there.
As you said originally in your question, the variables are the paint spec and the quality of the workmanship, I'd say the prep is 80% of the cost in terms of man hours if the job is done right, the quality of the prep is the difference between a good and bad respray, unless the sprayer is completely rubbish and messes up the actual painting too, I don't think you do that job for an easy life, it's a god forsaken smelly dirty job and all the guys I know that do it professionally say they do it for the buzz of creating something special and have immense pride in their work.
I'd love to do a bare shell respray on a K12, if I won the lottery, if I thought I could make money from it (which sadly I don't anymore) I'd set up a company that specialised in all things Micra's and K12's and offer bare shell resprays, gearbox, engine and rear disc conversions, interior re-trims etc etc, sort of something like Gas Monkeys in terms of their skills and abilities however concentrating just on Micra's and specialising K12's.