Hi Melinda,
Quite a number of thoughts come to mind, because I think people and horses are always problematic compositions.
First I think you need to decide in your own mind whether this is a figurative scene including people and horses, or a portrait. If it's to be a figurative scene, I can't help you very much (OK, well, not at all) with pricing. If it is to be a double portrait, you should charge according to what your published portraiture price states.
Horses are, as you know, extremely difficult to photograph. You will always need to be on a stepstool, or some type of elevated platform to deal with the horse, and the complexity is compounded because the size of an average adult is generally much smaller. Just the
head of the horse is so enormous that it is hard to derive a composition containing both horse and human where the human is not overwhelmed.
It is also my experience that serious riders/ competitors are extrememly particular about the photo showing the horse at its best - which means the ears need to look a certain way, the legs at trot or canter cannot be at an awkward angle - they're used to seeing horses in the stretch or confirmation that horse show photographers provide. Perhaps your clients think of their horses as pets, but likely not.
In Kristina and Pongo, I had several things working in my favor:
First, Pongo is actually a pony and is relatively short. He is also Kristina's pet.
Second, I had an excellent relationship with the parents (they already had another portrait from me), so that they trusted me to tell them what would work.
Linda Brandon loaned me her book,
"Atlas of Animal Anatomy" without which I would have had dreadful difficulty painting Pongo since I really needed to know the bone structure and underlying musculature of the horse's face.
I worked out very carefully three-value massing before starting the painting so I could keep the white Pongo from competing with Kristina.
I didn't charge extra for the horse. I don't charge extra for pets, as I guess I sort of think of them as props, and find them to take no more time than a background in filling square inches. I should probably rethink the pet-charge issue, though.
I have done a number of other equestrian paintings as well, simple portraits with riding habits, casual jeans holding a bridle, a child sitting on the paddock fence petting the horse (I was on a ladder for that one).
Small dogs and kittens are a lot more manageable. I suppose my advice is to really nail down with
you want to do, and make sure you have a meeting of the minds with your client. Once you decide, you might like to post some reference photos for critique or composition. (I am on the run but will move this thread to conceptualizing the painting - composition when I return.)
Good luck, Melinda!