Like specialty shirts. I'm a cheap bastard that shops clearance racks, thrift stores, and church sales so I have no idea what people normally pay for this stuff.
I'm using one of those print on demand services (Zazzle, in this case) to get some designs on shirts. The way this particular site works is the product has a base production cost and then you can set a royalty percentage on top of that that ends up being your cut. Default/minimum is 10% but you only make like $1 a shirt that way. I'm sitting here adjusting it trying to figure out whats not too expensive for people, but also where I get something for each sale. They will only cut you a check when you earn $25 or more so it would suck to sell like 20 shirts and still not get paid, you know?