Keys - $0.80 x 78
Diodes - $2-$3
3D printed shells - $240 from Shapeways. Cheaper from a friend with a makerbot
Keycaps - Cheapest when harvested from a board found at goodwill. Otherwise, $35 or so from WASD
Teensy++ - $24
I'll leave the "priceless" joke to the peanut gallery.
Could you ballpark what the cost to build one would be?(ignoring tools like the soldering iron?)