I'm not too sure how relevant MPH is as a factor. For me, at 55 years young, my night vision is a fraction of what it was when I was younger, so I am becoming more and more dependent on powerful lighting as the years go by.

How can you say LEDs are expensive? I bought a tactical 200 lumen red LED flashlight made out of machined aluminum for $10. Shipped from Hong Kong to my door.
Makes a great rear light, way stronger than any 'Superflash'. All you need to add are rechargable 18650 batteries, and you can get a charger and 4 quality protected 3000 mAh batteries for under 30 bucks.

The 900 lumen MagicShine from DealExtreme bike light is under $80 shipped to your door. While the overall build quality is not as good as some, it works well, and so far has been trouble free. That is about what my POS name brand 10W halogen cost me some hears back. The MagicShine has 3 times the run time, even on high, and you can't even see the halogen beam when it is on.
For my eyes and roads, the halogen was inadequate for anything over 8 mph, and probably even that was pushing it. Not a whole let better than the old flashlight thing I was doing as a kid (then again, the old night vision isn't what it was either).

Where can you come even close to those prices for halogen equipment?
I think it is more fair to say that there are manufacturers who are expensive, rather than technologies. I would guess that a lot of the name brand companies have cost structures built around the old (expensive) technologies, where the product component cost is now just a small part of the total.

Lastly, I've not tried HID lights on my bicycle, but I do do have an HID headlight on my dual sport motorcycle. Maybe what is used on the bicycles is substandard, or limited by the technology / batteries, but on my dual sport motorcycle, the high-beam comes on almost instantaneously, and gives much, much better light than the halogen that preceded in less than a second.