Prlce: NTE $20.
Bright: 2-5watt
Specifying output in "watts" can be misleading. Most of the maglite dropins for sale at Walmart, eg the "3 Watt LEDs" actually
do use 3 watts of power from your batteries. The problem is they are basically obsolete emitters -- they are less than one-half as efficient as some of the newer LEDs available today, which can be just as bright (measured in lumens, or overall light output weighted by human eye sensitivity) yet only draw <1.5 watts. Or they can draw 3 watts and be 2x brighter.
I saw one today at WM called the Luxion 3watt for maglite at $18 but I didn't look to see if it would work with the 6D. I don't want to spend a fortune tuning up an $24 maglite. I want to upgrade the 6D cause I use it as a deterrent for BG's and errant dogs when I walk my own dog at night. I have a Husky 2D but it's not long enough for the above use.
That particular Luxeon is an obsolete emitter like I mentioned earlier. I don't know of any Maglite "dropins" at brick-and-mortar stores that are very good, to be honest.
http://www.batteryjunction.com/tle-6k2.html -- this terralux dropin uses as SSC emitter (newer generation). It's $20, though may be somewhat more after shipping. It will both be brighter than the mag, and also draw substantially less power (eg longer battery runtime).
You will not be able to go any brighter than this terralux with a simple "bulb-type" dropin as any higher power LEDs will require heatsinking. The bulb holder on a Maglite however is completely insulated, so there is nowhere for this heat to go. In fact, the walmart 3 watt dropin you saw will only run at full output for a minute or two. After that, it will drastically dim itself in order to prevent from overheating.