I rarely do commissioned quilts because I've been told I charge too much when in reality I work for less than half of minimum wage. So once I break it down for them then they change their tune
. But I calculate approx how many hours I will be spending on the quilt and multiply that by how much I work per hour. So let's say I spend 55 hours on a quilt and I multiply that by 2.50 and get 137.50 on top of materials and quilting.
Hope this helps. Deciding how much to charge per hour is total personal preference. I, personally, am not in it for the money but for the process and what I get out of it.