An XM-L2 in a 501b body is about £8 from Lightmalls… you will need a cell or cells (I use reclaimed laptop cells, otherwise try Torchy[/url] on eBay), a charger (£5) and a mount (£2). I also have my charger on a timer just to be on the safe side.
On medium power this sort of light is plenty bright enough on the road and would need to be dipped (if you have the right mount you can just tip them) or switched to low power for oncoming traffic.
Personally I would always have 2 lights for ‘redundancy’ – and at £8 they are pretty cheap. I have one of these as my new lid light for MTB… though the XML-T6 501bs I have are still great lights.