Whatever you do, please put it in writing before you start. State what you will do: piece the top, sandwich, quilt & bind. List any materials you will have to purchase, such as backing fabric, batting and quilting thread. Get a non refundable 50% deposit on your time, plus any outlay for materials. Make copies, both sign & keep a copy.
As far as what to charge, you are providing a skilled service and there is wear & tear on your machine, so a minimum of $10/hour. If the quilt isn't cut out yet, be sure to include time for that. If it is cut, take a good look before you provide an estimate as an inexperienced quilter may have not cut accurately. If they cut too big, you will need to recut the pieces or the block. If too skimpy, you may have to revise the pattern. As far as estimating your time, I'd make a block using some scrap fabric keeping track of my time, then multiply by the number of blocks. You will need to add time for any sashing or borders. You are going to have to base your quilting & binding estimate on your previous experience.