Here's a thread about a big cheap light I made. It's basically the cost of the bulb. Ceramic sockets are about $1-2.
www.tomatoville.com/showthread.php?t=35099
That light will cover one flat, which is 1'x2'. So two would do a shelf for you. The bulb doesn't have to be that big. The bigger CFLs are getting harder to find. You can just put a splitter in the socket and screw in two 150 watt-equivalent bulbs if that is all you can find. I do spin the flats around every day or two so the plants on the outside don't lean inward too much.
Alternatively, you could do what Worth did and frame a lumber rack in the shape of your shelf. Untreated 2x2 pine is good for that. The sockets and lights are cheap. You can wire in as many as you desire.