Over the last 3 years I bought and tried a total of 9 relatively cheap lights off Deal Extreme (
Buy Diving Flashlight at Dealextreme with cheap price - DX ) the cheapest being 10US and the most expensive 45US. The 2 at 10US are great lights but not suitable for diving below 5 or 6m as the water pressure operates the button, so they were relegated to general use at home and in the car.
2 at 16US were complete garbage and never worked reliably, both finally suffering control circuit failures.
2 at 22US are great compact lights and are in regular use within recreational depths.
Out of the 3 at 40-45USD, one had an electronics/LED failure, the other 2 work fine but being heavier and larger end up being replaced on dive trips by the compact ones that cost 22USD and have the same light output.
Without exception all the lights I bought on this site needed to be completely stripped down before use, o-rings inspected and lubricated. Most of the lights arrived with dry o-rings and some with damaged or in one case a missing o-ring (although this was a redundant one as the light was supposed to have 2).
All the lights use 18650 or 26650 and I bought a number of spares of both types. I probably spent another 30US on batteries. I also spent about another 5US on a variety of o-rings used rebuilding the lights.
Some of these lights proved to be poor quality but most are going strong after a couple of years of use.
So overall I probably spent about 250USD and ended up with 4 good and well tested diving lights with high output on max setting (I've had more than one request from DMs leading night dives to reduce beam intensity) plus 2 general waterproof utility lights plus a number of reserve batteries and a variety of car and wall chargers.
PS: By stripped down, I mean completely stripped down. Not just the tail end where the batteries are inserted. I also opened up the front-end, lens seal, main body seal, rotating ring with the magnet for the magnetic switch, lubed with nautical grease the spring and ball arrangement that provides the click positioning of the rotating ring.