I’m unsure why this is difficult for so many beginners. You charge what you think it will take to make it worth your while.
Yes, the GAG handbook is a great resource, but as a student working on a freelance project, you’ll need to, as PrintDriver mentioned, adjust your rates accordingly.
Graphic design rates vary enormously depending on location, extent of services, reputation, etc. Some have set fees, but most freelancers start with an hourly rate they’d like to make. This doesn’t mean they charge the client by the hour, though. Instead, it means that once a project is well-defined, it’s a matter of quoting a price that reflects the amount of time and expense that will likely be put into it.
Just a word of warning: projects have a tendency to take longer than initially estimated. Once you’ve been doing this for a few years, experience makes accurate estimates easier. Just remember that clients (in general) have a tendency to delay, change their minds, ask for extras, throw obstacles in the way and otherwise inhibit efficiency and cost-effectiveness. In other words, if you think a project will take 10 hours to complete, it’s often more realistic to quote a price for 15.
Oh, yeah. Contracts. Make sure everything is in writing before proceeding — these written documents tend to head off all kinds of problems.