I wouldn't say Halo is really a 'typical exclusive' so you can't really base a comparison on it.
I mean, the 1st one was ported a long time after it was certain it wouldn't impact the 360's sales, the 2nd one was used to push vista and dx10.
The third one is likely in limbo until it's got a purpose.
Halo was originally being designed by Bungie to be pc/mac exclusive (MMO 3PS)... then Microsoft bought them out and made it xbox exclusive until gearbox(?) ported it over, not Bungie or Microsoft.
I'm sure you may be able to find some old video's of the Beta builds of halo before it was ported.. I know there are some video card commercials out there with halo before it was xbox exclusive. really, Halo was ported from PC to xbox, then ported to pc from xbox.. the game was just a mess in the end, I'm surprised it did so well.
-Halo before it was Halo (xbox)
http://www.youtube.com/watch?v=p7C65ogSLXM&feature=related
-Halo at Macworld 1999
http://www.youtube.com/watch?v=_KqFJ5pPu2g&feature=related
-Halo Geforce 2 trailer
http://www.youtube.com/watch?v=NBM9ywsJ_zY
In closing, don't ever follow a game by Microsoft, they will always disappoint, unless you just don't care what happens to games you get excited about early in, because Microsoft will change everything until it feels like it will sell a lot. I feel bad for Alan Wake.
-Tanshin