Maybe make a cheap version of your quilt as a test? That way you can see what it's like to make and if it seems good enough to sell. You could also show it to people (if your sales are local) so that people will understand exactly what they're getting. If they see both versions side by side and know the prices of each, who knows which they'll choose.
Would using interfacing but eliminating batting work better than the other way around? I've never made a T-shirt quilt and may not know what I'm talking about, but I would think the interfacing could add some of the warmth and loft lost from the batting.