It’s an HID:
http://dx.com/p/sdt-002-rechargeable-85w-hid-8500lm-white-flashlight-silver-black-179082
Is it most Lumen for the buck?
What are the reasons not to get one?
No, that is not the most expensive. The most expensive one is another HID light that there WAS in their stock, but now it appears as sold out. It was something between 300-400$.
But yes, among the ones that DX has in stock now, that may be the most expensive… XP
That question would be allot easier to answer if it was inverted, regardless here are three of my attempts:
1. You can probably find it a good 20% cheaper somewhere else
2. If your going to get a HID it is probably best to get one with a good reflector
2. Think of the opportunity cost… 
On this DX page are all their HID lights, the most expensive, over $500, is ‘sold out’. But the one you mentioned is the most expensive there.
Although this SST-90 torch is over $200.
Here is likely the LEAST lumen-per-$$$ light on DX, and it comes with the "blown-out pink sock" or "cat in heat" switch cover as a bonus:
BWAHAHAHAH! I wonder where you got that description. :bigsmile: 
Don’t waste your money on those type of HID’s with proprietary battery packs. The batteries usually are lousy and don’t last to long, assuming they work at all. Once the pack goes bad, the light is useless. True you may be able to purchase a new pack for it, but as the first, it won’t last long and is usually a waste of money.
Is it still the best bang for the lumens?
What would be a better lens?
Looking at something with about 200000 lux