In fact my father built my sister and I a playhouse that was at least 3x that size, with a front porch and windows with window boxes. It has power and running water (no bathroom though).
He built it with my grandpa who was terminally ill with cancer. And it is impossible to tell you all the great memories we made in that little blue house! Now it serves as the cutest potting shed in the history of the world. :p We've built several out buildings with family members since then. We recently finished a 400 square foot studio at our house and it ran us just short of 18k. But maybe things are just cheaper here in Louisiana.
Much less expensive:
Mr. Storage Sheds
http://www.mrstoragesheds.com/pricecabin.html
Lone Star Structures:
http://www.lonestarstructures.com/pricelist.html
Tuff Shed
http://www.tuffshed.com/
Tiny House Blog
http://tinyhouseblog.com/plans/
That 22k also includes the land, so it could be a good deal depending on real estate prices in the area.