Agree with TiRed on the main point.
Twenty quid on a back light is not a lot of money if you're using the bike on roads daily and like not being in hospital. There are lights I would probably not buy due to cost, but this bracket is fine for me. Front lights (not an issue) are much cheaper now than they ever were if you're prepared to put up with failings. My Chinese SolarStorm is awesome for the money but once you add in a quality charger that won't explode or kill your cells over time and quality battery pack that lasts for the advertised time (more than 1.5 hrs+) and holds its performance you're approaching +£70 or more on top of the £26 bargain and big name lights stop looking quite so expensive.
It would be if the light continually broke and you kept needing to buy more. And if you need to keep buying batteries the cost will go up also, although rechargeable batteries have issues with being left charged but not in use which is where my main question came from.
I've had cheap plastic row of LED's type lights before and I'd take some convincing to buy another. I've always killed them with weather or they just snapped and broke. The Lezyne I have has no signs other than some mud in places that will be a sod to clean out (but affects nothing other than cosmetic) to show it's been used every day since September and charged many times. At 70 lumen it's very bright. The Exposure is similar at about 75 lumen but people occasionally seem to have issue with crossthreading of the cap that doubles as the 'on' switch.
So, back to the original question and away from 'what light should i buy' as that isn't really what I was looking for help with: A light that is likely to sit for periods unused but you want to depend on when you need it. Rechargeable or disposable cells?