I don't like it when people try to save a few bucks by supplying their own garments. On the rare occasion when I do it at all, I discount the price by exactly what I would have paid for the same garments. Mark up is part of the profit for the job, and I keep that in. Most people realize there's no way to save money this way and let you get provide them.
I work by myself, and just having to let the person in to drop off the garments can waste half an hour by the time they explain everything you already knew and exchange pleasantries.
I won't print on expensive garments (football uniforms, etc) especially if they are polyester. The liability is greater than the profit. Even if you make it clear that you're not going to be responsible for damaged garments...If you print 40 polyester uniforms that cost $30 each, and they all bleed, I guarantee the customer isn't going to just say "Shucks"...