There has not been any anouncements that X1 platform will be any more than your existing service that continues to go up once or twice a year. Many posts say it's only for Triple Play subscribers for the early rollouts. There has not been any notices that the X1 DVR hardware will be anything additional in cost.
I'm in the Boston metro market. Comcast got me to sign up via a "win back" promotion, and I seem to have received X1 right off the bat. From what I can tell from my first bill, the DVR rental is $7 / month, and the service bundle (a triple play) looks standard as well. I don't see any additional X1 charge.
Seattle. Upgraded from double play to triple play in order to get the X1 box. Cost of the "upgrade" was $308.50 ! Sent the thing back, not that impressed. Lots of bugs that they have had over a year to fix and have yet to do so. Tech told me I would just have to live with the problems. No way !