This is far more a question than an answer, but maybe the best way for me to understand this stuff is to say what I think and get corrected. 
Sorry for the ramble.
Brightness is type of emitter and current, voltage I think varies a bit by emitter type, and higher current requires higher voltage at the emitter.
Voltage x current are watts (power), power is heat and places demands on the thermal dissipation of the pill and the whole flashlight.
Cost is from quantity, overall quality, size and machining of the body, lens, reflector, but also type of emitter and current control.
Two basic kinds of current control, current reduction (buck) (limits current to the emitter, could be as little as a resistor), and boost (powers the emitter at a higher level than a direct connection to the battery). Buck controllers are cheaper than boost, and higher current controllers cost more.
Quantity comes from what sells to the main stream customer, HF 9 LED flashlights must cost close to nothing to produce, They make them by the millions and have little inside of value, low cost emitters, I don't think any kind of real current control, maybe a resistor, and the bare minimum in cost of everything else. A single emitter with the same output as the 9 cheap ones would raise the cost, as would a current controller that ran off a single battery instead of three (higher voltage with 3 in series).
Special batteries have higher voltage than single AA or AAA cells, which allows the use of a cheaper style controller that need only limit current, not boost voltage.
An interesting example of costs is to compare the free HF 9 LED and the Sipik SK68, using Eneloops the final cost of both is about the same, with more light out of the SK68, smaller size, and better quality, due to the HF using 3x AAA and SK68 needing only one AA.
Sorry for the ramble.Brightness is type of emitter and current, voltage I think varies a bit by emitter type, and higher current requires higher voltage at the emitter.
Voltage x current are watts (power), power is heat and places demands on the thermal dissipation of the pill and the whole flashlight.
Cost is from quantity, overall quality, size and machining of the body, lens, reflector, but also type of emitter and current control.
Two basic kinds of current control, current reduction (buck) (limits current to the emitter, could be as little as a resistor), and boost (powers the emitter at a higher level than a direct connection to the battery). Buck controllers are cheaper than boost, and higher current controllers cost more.
Quantity comes from what sells to the main stream customer, HF 9 LED flashlights must cost close to nothing to produce, They make them by the millions and have little inside of value, low cost emitters, I don't think any kind of real current control, maybe a resistor, and the bare minimum in cost of everything else. A single emitter with the same output as the 9 cheap ones would raise the cost, as would a current controller that ran off a single battery instead of three (higher voltage with 3 in series).
Special batteries have higher voltage than single AA or AAA cells, which allows the use of a cheaper style controller that need only limit current, not boost voltage.
An interesting example of costs is to compare the free HF 9 LED and the Sipik SK68, using Eneloops the final cost of both is about the same, with more light out of the SK68, smaller size, and better quality, due to the HF using 3x AAA and SK68 needing only one AA.
