What would be the best 2000 to 5000 lumen flashlight to get under $75?

LED Flashlights – General Info 18650 Flashlights
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M6 or M6 with modifications. That will give you a nice set of modes, plenty of light, side clickie, and a safe multiple battery configuration.

http://www.fasttech.com/product/1519900

This is may put out more light, possibly a lot more, but rear clickie isn't as friendly to use, most likely has a terrible set of modes, and batteries in series isn't as safe.

There's also the Supfire L1.

It already has the output you want, but you can get 5000 lumens with a resistor mod. Maybe over 7000 lumens if you install XM-L2 U2's on direct bonded copper mcpcb's.

Even swapping those XM-L's to direct bonded copper stars will get you close to 7000 lumens.

So let's says $63 for the light, $10 for emitters, and a little time with a soldering iron.