The cheapest that I can think of would be a drop-in. Three of the most discussed are the new drop-ins that come with the newer maglites, terralux's ministart 5 and Gene's drop-in. I'm thinking the one that comes with the newer maglites and is available for individual sale as a drop-in module alone would be the cheapest though not the brightest solution. Poor heat managment leads this drop-in to decrease in brightness and will be noticeable if used for extended amounts of time. After that the terralux ministar 5 drop-in. It will not decrease in brightness, but I'm still a little unsure about the long term consequences of this because of its nearly equally as poor heat management. Next is Gene's drop-in. It will not decrease in brightness and has great heat management. It is also the brightest of the two drop-ins above.These are all simple, no modification needed upgrades. If you do a search for maglite mods you'll be able to find other ways to brighten up you maglite.