I suspect the $100 kit is indeed the XNA. (Which sounds like it's an abstract interface layer between C# and the "DirectX"s of PC and XBox 360).
This is a good move on MS's part, as they will probably use this model for future XBox consoles, and essentially remove the learning curve and allow very early prototyping on future microsoft consoles.
Of course, this won't include the hardware, or the licences, or Visual Studio (which I suppose you can arguably do without if you're familiar enough with .Net's command line compiler)
I'm sure your $100 gets their published standard and c# interface layer to support it.
This is a good move on MS's part, as they will probably use this model for future XBox consoles, and essentially remove the learning curve and allow very early prototyping on future microsoft consoles.
Of course, this won't include the hardware, or the licences, or Visual Studio (which I suppose you can arguably do without if you're familiar enough with .Net's command line compiler)
I'm sure your $100 gets their published standard and c# interface layer to support it.