I think on this kind of a service prices should vary considerably depending on the skill and experience of the trainer. If you are young and have only trained a few dogs, minimum wage might seem fair. It gives you a chance to get your hands on more dogs so that you can gain experience. Of course, the people are also expecting some sort of result. It may take you much longer to make those results if you are fairly "green". I think the best thing is to charge a small amount per hour and do all the training yourself (training the dog every day) so you do 1/2 hour to an hour per day with the dog. This way you control all the training and can learn as you go.
Once you are more experienced with the training, you can start coaching the people better. Then you can charge more per hour doing private lessons and meet with the people weekly to teach them how to train the dog. When you are still learning, it is really too much to teach someone else how to do what you are just figuring out for yourself. When you are really good, you'll also be able to handle this in a group class setting and still do a good job. Teaching groups effectively is very difficult- I rarely see trainers who really do a good job at this. It's definitely a job that should be left to the most experienced trainers.
Of course, if you are already really experienced (trained dozens of dogs through advanced obedience), then you should just see what your local competition is charging and use those rates as a guide. Jump in a start competing for the business. Of course, keep in mind that there are many skills as a dog trainer, one of them is sales ability. Your competition might be able to sell training at a higher rate than you can just based on that skill. If you are not comfortable selling, you might consider studying that end of the business, too.