There is really no set way to price design work, but there are different factors you should take into account when setting your price.

Experience should play a big role in your price.  Obviously, the more experience you have, the more people are going to be willing to pay you.  Brand identity from a well known company will cost more than a freelance designer working out of their home office.

Next to take into account is the type of work you are doing.  If you are doing a simple logo design, that will not cost as much as an interactive design or animation.

Creative jobs may not always pay the best.  The client will pay more for something that is more beneficial to their company, and if that means they need boring reports designed  they are going to be willing to pay more for that than a fancy animation.

In all reality, you have to find a price that you are comfortable with.  When you start out, you will probably have to start by charging a bit less than you want to.  Pricing is a struggle all freelancers face, but with time and experience, you will get more comfortable with it.

Seems like I may be having some luck with freelance artwork.  Just finished a "Pin the tail on the Yoshi" game for someone.  I will post photos soon.  It is so nice to be doing something I love, even if it is just something on the side.

I will have another project coming in soon, a charcoal drawing of a boxer.  People love their dogs, and I love drawing them.  If I can keep up these little projects they will help pay the bills quite nicely.  Looking forward to many more freelance opportunities.