First, consider how much your time is worth to you. Then look at any ancillary costs that you might incur while doing the job, such a printer costs, software, etc.
Then consider how long it takes you to do a file of the type they want you to do.
After that, its usually a negotiation.
I have asked for fees anywhere from $25/hr to $100/hr, depending upon the job.
So start with a figure a little higher than you consider your minimum. If they agree, then you make a little extra. If they balk, then you have some wiggle room.
I don't do that - I quote a fair price (which I estimate by working out as best I can how long the job will take and charging the same hourly rate to everyone). But I never add a negotiation factor. I don't get very job I quote for but I've never been asked to negotiate the price. I wonder if this is a UK/US thing.
(Sometimes if the job is much to expensive I've agreed a lower price for a reduced spec and less work.)
For me it was a personal choice as I could do so many different things.
A simple editing job or small research project is time consuming, but not taxing.
Doing detailed reviews or design/build projects takes more effort as well as time.
I have other friends that do just flat rate only, but they usually stay within a narrow range of services, so they stay within those types of projects.
