Once consideration is that the RaggTopp should only be applied to a absolute dry surface.
So if the top needs washing, to be honest, unless you live in a very hot, dry climate, you or the owner need to thoroughly wash the convertible top a day or two before your treat it with the protectant so the fabric can completely dry.
Having the owner do this means possibly getting a substandard cleaning job, I don't know the owner so I'm not passing judgment on their skill and abilities but I know all of you, much like me, are very thorough when washing anything we're going to work on and most non-detail oriented people will just do a quick wash and think they did a stellar job.
Of course, if the car is always garage kept you might be able to get away with vacuuming and then treating...
As for a price... I would say a low of $75.00 and in a perfect world, around $150.00...
Whether that's a good deal or not depends upon if you have to wash the top and then come back in a day or two...
Down and dirty detailing would be to just vacuum it, (or not), spray on the protectant, wait about 30 minutes and apply a second coat, then call it good. I'm not saying this is the best way or right way but I could see production detailers doing it this way...