Yeah, if you want to port your work to the X360, you have to drop that $100/yr. to do so. Don't think that Microsoft will open their console up for nothing.
You'll have to drop a hell of lot more than $100 if you want anyone other than your mom, dad and the vicar to play it. A modest five figure fee would be more like it, unless you've got something so amazing that Microsoft with front you the ESRB rating and so on. And I wouldn't hold your breath even then because XBLA is so inundated with submissions that even large indie studios with games already *on* XBLA can't get a response.