[Canadian Psycho]

Brightness, spread of the light, battery life, external quality all determine the costs of a flashlight. The higher priced ones are typically your dedicated weapon lights such as the Surefire Scout series. The dedicated weapon lights will cost more as they are specialized and built to handle the recoil of an actual firearm.

for a great set up without breaking the bank I would go with a Surefire 6P LED, with a remote pressure switch and a Rail mount for it. Simple, great quality light with lifetime warranty, works great both on and off your gun. Total would run you around $100

Tons of other options if you want to go with clone lights which offer the dedicated weapon light look without breaking the bank.