Might as well mention the alternatives to crazy expensive bicycle lights. Today, their is a great variety of powerful, battery-efficient, hand-held flashlights available.
For those who don't have a lot of dough to sink into a bicycle specific lighting kit, some of the available mounting hardware might be interesting.
First, is the old tried and true WALD flashlight holder. These used to be used in the day of the D-cell battery flashlight. Most of us probably don't remember the old paper wrapped acid-leaking short-life batteries of yesteryear. However, WALD still makes these mounting devices for bikes. Here is the deal, there still are flashlights out there that are made to hold in your hand and are big enough to fight off a rabid racoon attack. Today, however, these often have mongo LED bulbs and are powerful enough to flash signals to aliens in other galaxies. I have one and used an old rubber foamed handlebar grip to cover a skinny highbeam flashlight. It fits the WALD perfectly. The WALD weighs nothing and costs less than $6.00. It is sturdy and doesn't come loose. You can get these at Amazon.com or most bike shops can get them.
Next is the TwoFish flashlight holder. This is available at fenix-store.com and on Amazon. Amazon is $9.95 for one or Fenix-store offers a three pack for about $15.00
This is for smaller flashlights maybe 1/2" in diameter or less. It uses a rubber molded piece that fits the handlebars and the flashlight. The fasteners are velcro straps. This is an OK rig, but it is hard to keep the velcro tight enough to hold the flashlight in place. Eventually, your flashlight ends up hanging upside-down on the handlebar with the light pointing back at you.

Finally is the Ultra-Cost-Efficient tire inner-Tube flashlight holder. Using slices of old bicycle inner tubes as rubber bands, you slip the band around the front end of the flashlight, then wrap it around the handlebar, then slip the back end of the flashlight through the loop. I use big diameter mountain bike inner tubes. You use a folded slice of inner-tube between the flashlight and the handlbar to act as a support and prevent slipping. (for you collectors, YES, that is an original 1976 Schwinn Continental with original bar tape that to which the light is mounted).