yes 10% is usually normal. Some trainers won't charge. Mine wouldn't have charged if I wanted her to come up and see a horse I thought might be good to buy...but I knew a friend's trainer that would charge 10%
Are you getting your trainer to help because you don't know what to look for? Just want advice and comments? Honestly, I never really needed anyone's help picking out a horse. I made sure I knew alot about horse conformation, injuries, and so on that I could look at a horse and see how it was built, and such. You should know that even if you get a trainer.
I had a friend who owned a horse come up and take a look at my boy before I bought him, just to see if she liked him.
Mainly what trainers do is that they critique the horse as good as they can. Doing what a vet would do in a pre-purchase...but since they aren't vets, they wouldn't be able to find everything wrong with the horse.