Are Mobile Detailers "supposed" to be cheaper than detailing shops, in general ?

 

No; if anything they are more expensive. They come to you, you can go chill out while they are at your house, you can watch them, they come when you want them to. In contrast you get to drive to a shop, drop the car off, sit and do nothing in their waiting room for 6 hours (or catch a cab or have someone ride behind you) and they close their employee only door and say 'bye bye'.