I doubt you'd get more than £5 for it to be honest. Whether it's worth your while doing it is an entirely different story, if there are shops nearby that are likely to ask you to clean theirs then perhaps it is, if its just one in the middle of nowhere its probably not worth doing, that said as you say you drive past it anyway you might as well stop and do it, I've picked up at least £200 a months worth of work in the past from a single £5 job