Bottom line... it's a craps shoot.
Halide lamps for warehouse fixtures are around $20. The lamps themselves don't cost much to produce. What makes the halide lamps for aquariums so expensive is the the R&D to create the exact chemical compounds, then precisely dope the inside of the arc tube with them to produce quality color rendition and usable light.
A 400w lamp may be bright, but it's all a matter of how much of that light is useful to the livestock in the tank. If only 50% of the light produced by a 400w halide lamp is usable by the corals (and you can't tell by eye), then it's like you are lighting up the corals with a 200w lamp. The rest of the light amounts to wasted electricity and heat, and growth of stuff in the tank that can use the light (read: algae). It may be bright, but it's not useful.
The famous site where you bid on stuff has lots of lamps that are either noname, knockoffs, or factory seconds. You take a chance. The problem is that you may never know if you are getting a good lamp unless you have the equipment to actually test the lamp's emissions. When you buy an expensive lamp, you are paying for the name... more accurately, you are paying for trust in a lamp that has a known company and their reputation standing behind it.