If they're employing a consultant (or freelancer) on an hourly rate then they really, really are. Those people are not expected to learn on the job - they're employed to be an expert in the tech, to come in and get up to speed quickly, and to be extremely productive for a short time.

To answer the question: Pick an typical consultant rate for your location/skills/etc. Charge that for the first job. For your second job, add 10%. Stop adding the 10% when clients say no. You'll be surprised how high you can go.