It really depends on who is selling it, on Ebay. I have had good luck on Ebay. I picked up a 400 watt HPS/MH for $100 shipped. A Hydrofarm 2' four bulb T5 with bulbs for $60 shipped. An aircooled reflectior, for my 400watt HPS for $50 shipped.
Just look at the sellers feedback before bidding.
If you are talking about the cheap combo 400 watt HPS\MH units, I have one and it works great. The only problem I have had, was I had to retrofit the fixture to fit my aircooled reflector. But the ballast runs quiet and the bulbs burn bright.
P.S. I have had my light for only 2 weeks, and have been running it 20/4 flowering some autos. Not sure how it will be long term.