My thought is to never underestimate your time, talent and financial investment. The more you "lowball," the more you degrade the value of the entire industry. Making less money doesn't help the economy.
I'm not saying you need to rake the customer over the coals, just don't throw yourself (and us) in there.
I think $15 should be the minimum if you have any artistic content on your shirt. I figure that is a gross profit of about $8-$9. Then factor in your equipment overhead. And I guarantee that your overhead is underestimated. That's why many of us are doing this out of the garage instead of a shop.
$20 (for one color) is what I want to ask, but lowballers make me hesitant to demand it...unless more of us decide that our products ARE worth it. $25 should be what we want for multicolors.
Of course if you're pressing a cheap color transfer on thin shirts, maybe 25's too much.