I use the lightweight iron on stabilizer on my tshirt quilts, it works great, and if you hit Joann's for the After Thanksgiving blitz, they usually have the lightweight fuseable stabilizer in the 10 yard bundles for cheap. Otherwise, I use a 50-60% off coupon or a 40% and hit it on Senior discount day. So the cost isn't that high.
I made many tshirt quilts on commission at one time, but the bottom fell out of that enterprise when people felt that I was charging too much. (I wasn't charging enough!!). I had to sub-contract to have them machine quilted, but paying for the materials and batting isn't cheap either!!
I haven't made one in a few years now, and unless I am properly compensated for my work, I won't be making any more! Guess I'm getting to be a cranky quilter.