If you want cheap DIY, and you have a 4' tank, you can go with shop lights.
I got a couple of dual 4' T8 shoplights, gut them & use the parts to build my lighted canopy. Just make sure you get ones with electronic ballasts, and replace stock caps with waterproof endcaps if you don't have a glass top. I use regular Daylight T8's (since I am doing FW), and the whole thing costs maybe $50 for 128W of light. Now doing reef, you will need to get special bulbs at your lfs, which will add $20 per bulb.
For a bit more, you can get a PC kit. <Eg AH supplies's bright kits are ~$50 for 55W, plus cost for bulbs & shipping> The PC kits gives good bang for your bucks, and the reflectors are far better than the ones off a shoplight. If you look around, you can find some T5 kits for pretty good prices (~100 for a dual setup). The T5's gives out a bit more light (better reflector efficiency), but is essentially a straightened out PC. From looking at the prices, you can do better with retrofit kits than trying to buy the parts separately.