IMO the greatest bargain in bike lights isn't a bike light. It's the Duracell flashlights at places like Costco. My most recent set of 300 lumens lights cost less than $20.00 for 3. I mount a pair using an improvised block for stability, and a a long O-ring/ or rubber band to hold them in place. My usual arrangement is a pair of beams aimed one forward of the other. I keep the battery pack from the 3rd light with me as a backup.

The lights take 4 aaa, batteries, which need to be shopped hard to keep the cost down, or you can use NiMH rechargeables, but the light output will be lower, and life shorter. I run NiMH for the commute,and keep alkalines as my backups.

BTW- since these have a round cone beam, it's important to keep it down a bit, or aimed slightly to the right. That's never been a problem for me, but if an approaching car has their high beams on, I tap the forward light, and swing my bars a bit, and it never fails to get the desired response.