Mr.Dude just bought me this t-shirt for $10! We end up buying shirts from here all the time.
RiptApparel and TeeFury both sell 1 shirt a day for $10/$11. They are usually made with blank shirt from American Apparel, so the shirt material is soft and you can properly gauge the size (if you're used to American Apparel). You are able to select the shirt color if you think the design looks better that way. For instance, the awesome design above was modeled with a black shirt, but some people may like yellow or orange.
RiptApparel also sells the same design in hoodies, onsies and toddler sizes, a large print, a set of coasters and also as an iphone case. All very reasonably priced.
The only downside is that the t-shirt is only there that day. The next day it will be a different shirt and you will be unable to buy it again. They do sell baby shirts/onsies and hoodies sometimes, but not every time. TeeFury does sell two shirts a day a couple times a month, and they also have a Grab Bag even about once a month. The Grab Bag is very nice, as you only pay $5 and you get a random shirt from the "vault" (where all the shirts go after their day in the sun). Here is todays shirt design:
Shirtoid is also a great site. It links to the sites mentioned above (and more, although some are sites that sell for $26/shirt), but also find sites that sell old shirt designs. This is just a portion of their front page!
I've gotten a Little Shop of Horrors shirt, a ton of Sailor Moon shirts, a couple Labyrinth shirts and many more video game/tv show/movie shirts.
My sister bought me this shirt!
So there is a lot of cool stuff! I check in pretty often, and buy here and there. We got my Dad 5 Dr.Who shirts for his birthday (purchased throughout about 6 months). This is great, because my Dad never buys clothes, but also, is reasonable for gift giving. Just buy one or a few over a couple months and you won't break the bank. My Dad, Mr.Dude, sister and her fiance all love getting these shirts, so we all usually gets 1-3 each every Christmas.
I just thought I'd share. You can find so many. I recommend looking at Shirtoid and the galleries of each site to get a grasp of what kind of shirts they have.