you put your estimate in writing and have your customer sign it. There are some costs that you can be specific about (per inch cost, batting costs, etc) but you should have preprinted on the form what could be additional costs (if extra fabric required, custom design costs, etc). This way there can be no argument afterwards about what it cost. This is double (or triple) important if you are dealing with a friend or family member. If you choose to give them a deal when it's done, go for it. But at least you have approached the project in a business manner.