It appears they will have three different versions of Halo 3 next week. There's the plain vanilla version, with no extras, at the "normal" $60 price point. For another ten bucks, you can get the limited edition, which includes a DVD with making of stuff. For an extra $70, that's $130 total, you can get the "legendary" edition. It has the game, the behind the scenes stuff, a bunch of Red vs. Blue episodes, all of the cut scenes from all three games (including commentary from the development team), a featurette about working at Bungie, and best of all (kidding), a giant shiny Master Chief head that holds it all. How fun is that?
The fan in me wants the big one because of the composited story elements brought into one place, but that sure is expensive. And I'm not sure where I'd put the big head either. :)
I have a soft spot for geeky toys (which is why I have the legendary edition on pre-order and a big daddy protecting my cube at work). You forgot to mention that with the limited & legendary editions, they come with A/V calibration tools on a DVD. I have no idea how these compare to the DVD essentials version, but I would say (speaking of the limited) it is worth the extra $10. Ohh, also the limited comes with an art book (the legendary does not).