Estimate the number of hours you spent and charge at least $10/ hour. If you purchased anything for the project, make sure you charge for that too. If you took fabric from your stash, charge the going LQS rate, not the on sale price you actually paid. You will need to replace your stash at the going rate! Be sure to charge for travel time to purchase any materials.
And please, please, please, if you do this again, put an estimate in writing before you start and get a deposit!