Driving POWER LEDs ...

Using Arduino LEDs and Multiplexing
6

http://www.ebay.com/itm/10x-Power-Driver-MR16-1x-3-Watt-LED-Light-Lamp-12V-/300601422018?pt=LH_DefaultDomain_0&hash=item45fd3db0c2

These are about $1 per driver and you can probably drive two leds in series with each driver. You will not be able to build anything cheaper.

Lefty