I just did a t-shirt quilt for a friend too....it was 16 shirts and fit a queen size bed. She bought the fabric, thread and interfacing (about $140) I was in the same situation.."what to charge". I probably had about 50-60 hours in it and had in done in just over 2 weeks. (She wanted it for Christmas...there was no rush...I just put my nose to the grindstone and got it done...other projects on the burners) I looked on Etsy and they were charging on average $350-400.00 (they supplied the materials. I finally decided on $200....she was THRILLED. I'm sure whatever you decide they will be satisfied just knowing that someone they knew did it. Good luck.