I run a silk screen press. Unless your buying a minimum of say at least 25 you aren't going to find them for much less than 20 per. It takes far too much time to transfer the design onto the screen and then to print, plus the cost of the shirt. quiet honestly its a waste of time to create a screen to print just one shirt.
If you want 20 shirts i got you for 12 per if you come up with the design yourself.