This mistake can kill a writer just as surely as any of the others. If you accept a project thinking that it will take you only 8 hours and it winds up taking you 24, well, you're not making much money (or at least not as much as you should). There's really only one way to avoid this mistake. You have to read everything about a project, and read it carefully.
Before you read anything, determine what your hourly rate is. I charge a minimum of $60/hour of work in most cases. I'll work for cheaper in the cases of charities or other worthy causes, but I won't work for free (see the second mistake). That $60/hour (or more, depending on the project) includes research, a draft version that is approved by the client, revisions, editing, and the production of a final polished version. When I just started out, I charged less. As I gained experience, my price went up (just as the time it takes me to complete a given project went down, so clients ended up paying the same anyway).
Once you know what you charge per hour, figure out how many hours the project in question will take you. This requries a little experience and you'll probably get it wrong a few times, so get to know yourself and your ability to complete a project. I can complete a 1000-word article, start to finish, in about 1 hour, if the subject matter is at least a litle familiar to me. Subjects I'm not versed in may take 2 hours. If I'm ghostwriting an entire book ... well, that takes more than just a few hours. My price always reflects how much time has to go into a project.
When your quoting a price to a client, know exactly what you're promising. Work out exactly how much work you'll be doing and have it specified in a contract signed by both you and the client. You'll probably make a mistake once in a while and have to just eat the cost yourself. But a contract helps you to see how much work you're committing to. It also serves as a barrier for when a client tries to pile on more work without providing you with additional compensation. This will happen, so learn to stick to your contracts.
