There are a lot of people building their own fixtures. Google "DIY aquarium lighting" and you should find some pretty helpful threads on the other big forums.
To give you an idea of what it will cost: http://www.rapidled.com/servlet/StoreFront
Most people are using one LED for every 15-20 square inches of tank surface. In addition to the LEDs themselves you may or may not need optics, but you will also need a heatsink, fans, and powersupply.
-Justin