I have made several t-shirt quilts for other people. The most I have charged was $250 for approximately a twin size. Several people never contacted me again when they heard the price. I am don't use stabilizer or interfacing unless the t-shirt is very thin. I also might use stabilizer if the shirt is mesh (like a basketball jersey). I quilt each block differently. I use the 'Too Cool' layout method.The blocks are not all the same size. They are in increments of 4 inches. for example 4x4, 4x8, 4x12, 4x 16 - up to 16 x 16. The block size is determined by the size of the design/logo on the shirt. I do not use sashing strips. I do put a cotton border on the quilt top and cotton backing. Here is a link to a post showing one of my T-shirt quilts.
http://www.quiltingboard.com/main-f1...e-t171597.html - look for the entry by lakekids.