I was reviewing an invoice where I quilted a T-shirt quilt. It took me 1.5 hours to pin baste a queen size quilt so I would go more with 3.5 to 4 hrs to get the quilt ready and do the tying. I think you have underestimated the time to fuse, cut and attach the blocks together with the sashing. It is also going to take time to lay out the shirts once they are cut. I don't think you can do it for under 20 hours. Be sure that this friend of a friend knows just how heavy these quilts can be when completed. I would suggest that you tell her your estimate is for $200.00 and if you find you can do it in less time, than pass on the savings onto your customers. Help her find the sales so she can also save on fabric and thread cost. I bought my thread at wholesale prices and I passed the savings to my customers
As for labor cost, charge what YOU feel comfortable in doing. Some of us really DO NEED the extra money in these tough economic times. I am going to be selling Mug Rugs this weekend with most of them for $8.00 when it takes me almost hour to do the hand stitching. I value my work but I need the money to be able to take my Bernina in for the long overdue servicing that it deserves. Good Luck