I didn't think of this in enough time to set up any TH shirts, so you'd have to get your own custom made. I suspect this would cost between $10 and $20, most places, whereas if I were selling official shirts they'd end up costing closer to the $20 side anyway. So this way you're probably paying less, and you get a $10 donation reward in exchange. That's cost-effective, if a bit more hassle on your part.
Approved slogans currently include:
My other shirt is a cape.
Superheroes. No vampires.
If you've got another slogan you'd like to use, check with me ahead of time to have it approved.
Photo evidence would need to be posted here or sent to me privately within one week of the end of Comic-con to qualify. Pictures would need to show you at the convention, wearing the shirt.