First, let me just say that I have made several t-shirt quilts and they are NOT hard to make at all. As soon as you cut the blocks out of the front (or back) of the t-shirt and iron on the interfacing, they become super easy to use for a quit. When my best friend died of breast cancer a few years ago, I took all of her t-shirts and sweatshirts and made quilts for her kids. I'm not a real experienced quilter, so if I could do it, anyone can. I made all of the blocks the same size so that I didn't have to do any calculations.

Secondly, because you are using t-shirts, there will be very little fabric cost. I put sashing between the blocks and a border or two around the quilts. So your biggest cost will be for the backing and batting.

Lastly, I would only do a lap sized quilt. If your friend can't choose just 12-16 t-shirts for the front then you could always use some for the backing. That would save fabric cost. Can't comment on what to charge. I give all my quilts away as gifts.

Good luck! I think you would enjoy making this!