I recently picked up a drop in led bulb for a 2 D cell maglite.

It's called the SMJLED, costs about 8 dollars and will run for 50 hours until it reaches half brightness, and by some accounts on CPF from those who've tested it, gives another 4 days of usable light afterwards.

The bulb will work in any light that has a 'pr' base, so that includes many $1 specials - you don't need to drop it in a more expensive light if you don't want. PR bulbs can also be found in several AA lights if you want a smaller light, but it sacrifices some runtime.

Combine that with a 12 pack of D cells from Costco, and you have (conservatively) a month of light if used constantly, for under $50, less if you don't use a Mag.

Definitely a staple for my bug-in stuff.