you will get lots of answers most of them will be "I charge enough that they don't want it done"...for many reasons, it is NOT something that is fun to do!
IF you think you still want to do this, it is the actual cost of all materials, even if you don't use all of thread, etc..you charge what it cost you to buy them. INclude rotary blade, machine needles, etc. Then you charge an hourly amount for everything, washing, ironing, cutting, sewing, binding...keep a record book and every moment you spend working on this gets written down. DO a trial run on a quilt you are making right now....then look at the total and ask yourself this...would YOU pay that much for a quilt? IF the answer is NO, then you will know not to take on this job!
The last quilt I was asked to commission was a King size double wedding ring, for materials, supplies, quilting, and labor the BARE figure was $1,200.00! And that paying myself $10 per hour! THe LAQ was going to make more than I was! Needless to say, the person decided this was not the wedding gift she wanted to give her daughter!
IF you do decide you just have to take this on, remember you also have to charge and PAY taxes on it. YOu must get 50% up front, non-refundable once the fabrics are ordered! This is THE most important thing or you will get burned when they cancel the wedding, change the color theme in nursery, you name it......
Good luck!