A couple of additional thoughts: Eddie Justo has a good point–what you charge should be based on your actual cost of producing the project. Multiply that by a markup, and you have a rough idea of what you can charge. Personally, I wouldn't charge less than a 100% markup under any circumstances. In the ad agency world, a markup of 400% is not considered unusual for high-quality work.
Under this approach, project estimation and budgeting is critical. Make certain that your client knows what is and isn't included in your proposal, and get everything in writing! Make sure that the client understands what services are included in your quote–how many days' shooting, how many cuts of the deliverable, and so on. Otherwise, they will break you with forty days and forty nights of shooting and a dozen cuts of the film. All for a thirty-second commercial.
