in order to get a shirt, that they give away for free at their booths all over the place?
+ $20/mo to get a shirt is way overpriced...at that price, I might as well get a shirt from threadless that I like instead of "randomstartup.com!" one.
The business model should be flipped. Startups should pay you...to distribute shirts for free. And allow them to make it local....i.e. 300 shirts to people in los angeles.
Cost wise just charge them the same $20 per shirt. $10K for a one time marketing cost is nothing really. And getting 500 people to wear shirts would be worth it.