September 3, 2007
High-intensity discharge or HID headlights shoot a high-voltage surge through electrodes, thereby stimulating a gas (in most cases xenon) and disintegrating metallic salts. Those salts maintain the surge, and thus produce a light that is two or three times more powerful than standard halogen bulbs at a fraction of the energy spent. They also last much longer, and provide augmented peripheral vision (which is always helpful when a deer is about jump in front of your car). Like fluorescent lamps, HID lamps require a ballast to start and maintain their arcs this why the cost is as high as it is.
As they burn/wear out, many HID lamps go into what is known as cycling. a Dying set will start up at low voltage but as they heat up during operation, the internal gas pressure rises and more and more voltage is required to maintain it's ignition source because of this maintaining voltage for the HID eventually rises to exceed the voltage provided by the ballast. As the lamp heats to this point, the lamp goes out. Eventually, the lamp cools down again, the gas pressure is reduced, and the ballast can once again cause the light ti re-ignite. The effect of this is that the lamp glows for a while and then goes out, repeatedly.
Replacement to my understanding is just like any other headlight as in you still have a bulb, assuming that the ballast doesn't go out on you then you simply replace the bulb. Replacements I've seen average $150.00+ for the bulbs but most manufactures state that their bulbs will last 2500 hours+ and claim that they will last for the lifetime of the vehicle.
Also with a little research you can find kits for much cheaper than you say at Ford. Mrbodykit as an example sell them for around $215.00 for a low beam only kit and Americanmuscle sells them for around $220.00 for low beam only and $280.00 for dual beam kits; they also sell a fog lamp conversion kit for about $220.00 so you don't have to pay $500+.
Hope that helps!