I am currently doing this and in the process of building a detail shop. Here are my views;
First, I believe this to be another needed profit center. Materials and tools are relatively inexpensive and basic process make it fairly easy to perform basic tasks such as interior cleaning, shampooing, waxing etc. I think when I'm up and running $300 or more per day avg is realistic based on my location. Doing a completely clean, shampoo and wax on an avg car for $125 and taking 2 to 3 hrs to do is not bad money.
Second, using an existing bay is expensive (unless the bay in not used). My bays are new and fully loaded with almost every option, they cost about $75k each, so I want to leave them to what they were designed for.
Lastly, labour...you need people to do this and that can be a pain. We have an incentive program here where the person performing the work gets a cut.
BigLeo