A pre-qualification letter is free to obtain. It lets you know roughly how much you can borrow and requieres only your basic financial information.
You may also like to get a preapproval letter from your local home mortgage consultant, and prevent delays with your financing and purchase offer letter. A preapproval shows sellers that you are serious about buying a home. It is more powerful than a pre-qualificaiton letter because it involves a more detailed look into your financial background.