ThinkGeek are having a grab bag sale with tees from $5.99. That’s pretty cheap but ladies and 2XL and bigger sizes will cost you a bit more, up to $8.99.
Read the details though to see if it is for you. I wouldn’t recommend ordering multiple shirts unless you want multiple shirts with the same design as that is what they say you should expect. It’s a bit lame in my opinion but there you go.