I think it really depends on what the exercise is that's being demonstrated. I can see that being very helpful in some circumstances, but completely unnecessary in others. One of my biggest complaints with training videos is that they rarely show a horse that hasn't done the exercise AT ALL. So, when you go to work with your own horse you don't always know how to get started.
I don't think you can charge more because there are more horses in the video- that's really just not how it works. The amount you can charge is based largely off the trainer's reputation and how much other, similar DVDs cost. The costs to produce the DVD might go up a little bit if it's longer (particularly if you're hiring a professional to edit it) but the costs of stamping each DVD shouldn't vary much. People might pay more if it's a multi-DVD set, but generally won't pay more for twice as much content on one DVD (weird how people work sometimes).