I'm going to just poll a few ideas out of my hindquarters again. What that $100 will likely get is just straight software, without a single cable or anything. Development won't be 360-specific or PC-specific when written, with the XDL doing the porting from one platform to the other when compiled.
I'm guessing that the software won't be the only thing you'd need to write code, acting instead with MS's existing programming software for PCs. In other words, that new $100 package will be a plugin for what is already available.
Programs such as XBMC, Gentoox, and so forth could still be created with this utility. Distribution would likely be through MS's Live network, and may require a registration fee for the approval process, pass or fail. Emulators will likely have little chance of getting through, since anything for playing Playstation or Dreamcast or whatever titles will be a legal mess outside of the scope of the program.
The process for getting your code released on the PC side would be more straightforward, and likely cheaper, to the point that the new development kit would be pointless. But on the 360 side, it'll promote the Shareware market. If my guesses are correct, the Offline XBox Arcade market will get the greatest benefit from this.
So unless I'm just being needlessly pessimistic, I'll be modding my 360 with the rest of you eventually.