Much of the money is the prep, so the less they need to do the cheaper it should be. A lot of places aren't keen on relying on customer work tho.
Biggest problem doing it diy, is getting a clean dust free place to do it, where flies don't get to the paint, and the equipment to spray proper paint without losing your lungs in the process. Plus the skill required!
For a detailing co to do a wet sand and polish would be more than a couple hundred quid (for a decent one!)
Usually a couple hundred gets a machine polish to rectify minor scratches and swirls. A full blown detail with wet sand and polish to a high standard would be closer to a grand than you'd think!