The solarbotics Ardweeny http://www.solarbotics.com/products/kardw/ is just under 10$, hard to beat. Even if you go for total DIY, you still have to buy some kind of PCB / perfboard / stripboard, the Atmega Chip, and a handfull of components.
The Ardweeny kit kan be soldered up in less than half an hour.
I bought a couple of them and they work just fine.
Of course they don't have the USB interface so you need something like this http://www.solarbotics.com/products/50512/ to program them, or you could take the Atmega Chip and insert it in an ordinary Arduino and do the programming.
There are other small Arduino clones, the bareboard arduino, the boarduino and lots of others that are just as good as the Ardweeny.