Ive got a lot of respect and admiration for those of you that detail this professionally. I think the most important thing is that you charge based on the time\engery and the level of attention to detail.
If I were to detail my entire car to my personal level of satisfaction it would take me 30+ hours, at least. Id estimate it takes me roughly five hours each for the following: wheels off, engine bay, undercarriage, interior, exterior, tape + compound, polish + clean paint + LSP (including setup, cleanup, and putting away the tools & products). I also do maintenance while I detail or vice versa.
I dont know how practical this would be but you could offer fluid changes during a detail (most of the time is spent waiting for fluids to drain when you could be detailing). I did my differentials, transfer case, and motor oil & filter during a "wheels off detail" last year in a friends driveway. I bought everything there with me mobile detailer/mechanic style (so it can be done, IMHO). The dealership charges $700 and up for the fluids I changed to be changed (cost me $320 in fluids). Anyways just and idea... If I detailed professionally Id offer high-end clients to have their fluids changed and to be set off for lab analysis (thats what I do as proof and personal reassurance). Blackstone Laboratories is fairly cheap at about $7 per fluid analysis.