I've heard the rule of "3" too.... a third for materials, a third for time, and a third for profit.
Unfortunately, when crafts are involved this figure can be far from accurate. So much depends on the pattern, etc.
We have a term here called "mate's rates", which means you charge less for a friend than you would for the general public. I am doing a single bed quilt for a very good friend, and I will charge her only for the materials, whereas another being done for an "aquaintance" will be charged for my time as well as materials.
I guess what I'm trying to say is it's completely up to you! LOL