I'm sure it'll vary quite a bit since it isn't that common. You are basically circumventing their business this way (e.g. you are providing your own stuff, which they generally provide and make profit on)... some people won't care, but this could annoy some companies as well. (Sort of like providing your own parts when getting your car repaired, though not a perfect comparison.)
So, some might not care and let you use it for free, some might charge a nomial fee, and others might not let you use it at all =) $1-4 per shirt would probably be what you're looking at if you are getting charged.