Charging for web design service?

This is something I’ve never been quite sure how to do. If I create a website for a small buisness, and make lets say one update a month, what would be a decent charge for that buisness?

I’m thinking I would first ask for an “up front” cost of about $100 for the design and what not, and afterwards about $250 for completion of the entire website. Then for the monthly maintenance and updates, I would ask for $40 a month. Is that too much?! or too little?