The MC HCK costs $5 to build, and you can make it in your home.
For those of you who would enjoy playing with a development board but haven't quite decided on one yet, let me suggest this fun approach. You can make your own little board based on a Freescale MK20DX32VLF5 for roughly $5. It is called MC HCK and it is completely open-source.
MC HCK, short for Microcontroller Hack and pronounced "McHack," was designed to be super cheap and super easy to make at home. The MC HCK project, community-based and open source, really nails the super cheap part, but unless you're used to soldering surface mount parts, you may find the easy to assemble part debatable.
However, "the community" has done a fantastic job of making every other aspect easy. You can download the pcb files from MC HCK's website and send them off for production. The project's website even lists the people they recommend along with the cost (10 for $10). You can get the microcontroller from Freescale directly at five for $5 as a sample. After that, there is a very detailed parts list, including optional items with links to places where you can buy the parts cheaply.
Once you've assembled everything -- and the MC HCK website shows a very detailed slideshow of that as well -- you can begin to program. Their wiki has full instructions and examples to keep you going. In the end, you're left with a full-featured, tiny development board that has a USB interface that breaks down to roughly $5 per board.