Heya Pro Shine,
I'll share my experience in case it might help you:
When I first got started, I wanted to go with the 'hourly' route but quickly found that (at least around here) most of my customers greatly preferred a "package" with a set price-point. So what I did when I was first starting out was to do a few vehicles in each size category just to get an approximate baseline of how much time it would take for what I was going to include in each particular "package".
After learning the approximate amount of time that I would need for each service that I offered, I put together various packages figuring in how much I wanted to get paid per hour that I would need to do the job. Now, keep in mind, you always need to compare what you'd like to be paid and what you feel you deserve to be paid against what your local market and client base can afford to pay. It's not always going to come to the same figure. I found that, in my area, I could get approximately $20/hr. for each job. Others may command more or less depending on their level of expertise, their client base and the shape of the economy where they live.
I also note on my price menu that there will be additional costs for extremely filthy vehicles or excessive paint defects (assuming the customer wants a complete correction). And, as others have mentioned above, I never give a final quote without seeing the vehicle.
If I feel that, due to the condition of their vehicle, I won't be able to correct certain issues completely (typically due to time constraints with the package price point that they have selected) I will let them know and then offer to either A. charge them $20/hour beyond the amount of hours I have figured into the package that they have selected, or B. let them know that I will do the best job possible with the amount of hours that I can spend on that package and inform them of what to expect. Many customers will choose option "B" if it is the first time they are hiring me. When they do, I typically find that my standards are a lot higher than theirs and they are completely happy with the job I've done even though I would have spent a few more hours if it were my vehicle. I also find that some customers will go with the second option then to bring their vehicle back 6 or 12 months later and that I'm able to easily do the detail up to my standards within the package timeframe because I've already taken care of the major issues the first time that I worked on the vehicle.
So, in summary, it's a matter of determining what your customers want and the rate that they will pay for your services and then set your price points and service plans accordingly.
I hope this helps a bit!
-J
P.S. I work out of my garage rather than a shop. I don't have to pay employees, rent, etc. So of course, someone would want to take that into consideration when determining the amount that they need to charge.