I put one together after spending about an hour reading the wiki. The required parts are: 1 screen, 1 arduino, 1 transistor, 2 diodes, 3 resistors, 3 buttons, some wire, solder, and a soldering iron. Parts are detailed here:
MPGuino - EcoModder
I found everything except the screen and arduino for under 10 cents, but the cost of tools and a spool of wire added to my build cost.