If someone pays your to create a font based on their handwriting who owns the copyright to the font?

That depends on the terms of your agreement with your client. Such things should be discussed with your client before the start of the project. You should have a signed contract with all the terms that you want to be included as well as those of the client. Of course all the terms should be agreeable to both sides. If you do not have a signed agreement, then everything is open to interpretation and it might not be acceptable to both parties