Since we live in the middle of nowhere, it is pretty common for us to do an exam for a horse here that is being sold to someone else. At least, if the horse passes, it give the prospective buyer a good feeling if the horse has passed the vet exam that they are buying a sound horse, or they know that they might be buying an unsound horse if it doesn't pass.
I can't believe that many vets would risk a problem with their license by giving a vet check on a clients horse a pass on an unsound horse.
Call the vet you are thinking of using and ask these questions. It is a common question and I am sure the receptionist (or whoever answers the phone) will be able to give you an idea. The price varies depending on what you have the vet do during the exam.
The vet you plan to use for your horse care might be able to offer you the name of a vet closer to the horse's location if your vet is not willing to travel that far.
My advice would be to contact the veterinarian you are thinking of using and ask directly as there is a wide range of things that can be done and prices to go along with that. As a veterinarian, I always recommend my clients ride the horse a few times before having a prepurchase as you'll have a better understanding of the horse. I also, always recommend a prepurchase exam regardless of cost. The purchase of the horse is the least expensive part of owning a horse, and things you find on the prepurchase will gie you a better understanding of what that horse may need in the future to perform the task you are looking for it to do. A prepurchase may save you thousands in future costs by finding potential problems.