To be honest, you almost have to do a few jobs simply guessing your worth before you finally figure out what your worth really is. The price you charge should be based on a balance of expertise, quality, and the costs you must recoup. Throw a number at the agent. If he takes you up on your fare, then do the job. At the end of the job, figure out if it was worth the money--if not, charge more next time. ;-)