If you are only breaking even on these shirts then you are doing something wrong. I've had custom shirts made in bulk a few times and with a design like yours you shouldn't pay any more than $10 each. You could even find them for less depending on the brand of shirt and size of your order. So either you are lying, or Know is even more accurate in his description of you.
That's cool; I'll have some at the festival. Ten bucks.
I don't know why but I would actually consider this. $10 is where I usually draw the line at crappy t-shirts unless the artist has to pay for the use of a trademark, which is clearly not the case here.