Yep, I blame the 'Go Compare' culture that is hammered into us on a daily basis, suppose that is why you have to sell yourself, equipment and method. It could be someone with a George that whips around the house in an hour and sees it as an easy 30 quid. Its an end of tenancy, very often all they are interested in is the receipt to say its been cleaned!
They might even be basing their pricing on one of those 'leaflet through the door companies' and think that is the going rate, they might even be charging extra for each spot and mark, we just don't know!