Where the hell did you find Doom3 for $100!!!??? It's $50 bucks at BestBuy, and if that ain't THEE expensive source, I don't know what is!
Anyway, there's the Doom3 release version, then there's a collectors/limited-ed Doom3 box, coming out in Oct (I believe) which has some "extra goodies" in it. I can only assume that means perhaps an editor, add-on multiplay crap, etc, etc.
The PC gaming world talks...but they don't confirm before press.