The price should have been negotiated BEFORE the project was begun. Did you talk about the cost of materials? Labor cost? At this point, submit to your client a showing of materials cost, and add an amount that he will pay. I'm thinking about $70. Most people think in terms of an hourly wage: so how many hours x your hourly rate. Good luck.
If you want to do MORE of this kind of special custom work, your Reputation for fairness might be more valuable than the dollars gained. Be wise, don't make an enemy.