Pricing is about two things -- experience and confidence. You have to have the experience to warrant the price and you have to have the confidence to ask for that price. Here is my personal opinion on what is fair market value:

0 to 1 yrs experience and a weak portfolio = free to $10/hour
1 to 2 yrs experience and a strong portfolio = $25/hour to $35/hour
2 to 4 yrs experience and rock solid portfolio along with strong client list = $45/hour to $65/hour
More than 4yrs experience, inspiration worthy portfolio along with a strong client list = $80+/hour to {the sky is the limit}.

Obviously there are many factors to take into account. The "opportunity cost" should be evaluated in every situation. You need to know your client too. What sort of budget do you think they typically work with? You obviously want to get the most money possible without losing the gig to a cheaper designer. Always go high (not so high you scare them away) and feel their reaction. You should always have a plan B ready in the event the client is turned off by the rate. Explain to them the logic of the rate (there should be logic!!) and demonstrate how you could complete the same project at a lower, more suitable, rate. Things such as offering two mocks instead of four, delivering in 2 weeks instead of 1, etc.

Disclaimer: I hate attributing years-worked to experience (just an easy way to do it). I acknolwedge that some designers work in an environment that produces more work in 6 months than others do in 6 years.