do you want a bulb and ballast because a 600 watt fixture will be more than $20. you may find a bulb for twenty but probably not the set. good luck though m8.
www.dchydro.com has some of the cheapest bulbs Ive seen. They have two diff Plantmax 400w MH for under 20. I bought one since price is an issue right now. How good they are. I dont really know. Not too many people seem to use them.