Here's my $.02 (warning, it's long):
Quilting is expensive. The "quilts" you see at Target or Bed, Bath & Beyond (for example) are usually made in China, of poor quality fabric, and fall apart in no time. What people think is that I can do a quilt for <$50.00. When I quote them a fair price (starting with fabric which is $10-13/yard and up, thread, rotary blade, machine needles, batting, backing, etc., and not including my labor) they blanch. Then I tell them how much a long-arm quilter will charge to finish the quilt. And then I ask them what they are willing to pay for my labor. That usually stops people.
As for t-shirt quilts, the t-shirts have to be backed with a stabilizer of some sort. That's labor intensive. So first the shirt has to be fussy cut correctly, then backed. And you are dealing with different fabric blends (not all 100% cotton) which is a pain. You have to figure in the cost of the fabric for the front (including whether or not you're going to use setting stones when framing each t-shirt, borders, and binding).You don't simply cut the shirts apart, fuse them, and stitch them together. Oh, okay - you can do that but it's gonna look like crap. If you want to cut the front of the shirt up, and stitch that to the back of the shirt, you're adding another extra step.
Important point: there's the whole issue of
the cost of a long-arm quilter. Disclaimer - I don't know anything about the person in the link, I simply included that to give you an idea of what I'm talking about. Long-arm quilting machines are outrageously expensive. Of course, you can just stitch in the ditch, and that'll probably be adequate. It won't make the quilt very stable, however.
That said, there are quilting groups on Facebook (I belong to several) and if you want, I'll post there and get you some names. With any luck you'll find someone local. Otherwise you're also going to figure in the cost of shipping.
How to fairly price a quilt is a huge hot button among quilters. Gee, you couldn't tell that, could you?