I use these mounts and have them on most of my bikes (which reminds me, it's time to buy a few more for the ever-expanding fleet...) The price is pretty good, but you'd still need to add batteries and a charger, so figure on another $15 - $20. I've been running the same 4-pack of batteries that I bought a few years ago, and have upgraded lights a few times as the tech improved.

You might prefer to go with one of the bike-specific lights, which is only a few bucks more and has longer runtime, but I like the convenience of being able to pop the light out of the bracket easily when I lock up the bike, or to transfer it to a different bike.