no not really HTC is not discontinuing the One X
usually the previous flagship keeps going for another year, how else you think they will cover it's software development
its 2012 smartphone of the year according to most media, before the One it's their most highly acclaimed device ever, they've worked so hard to perfect it hw and sw, so no its far from done (at least in unlocked international form)
supplies will lessen ofcourse as they focus their production on the One but i don't think anything is being discontinued here, the One S and the rest of the bunch however...
We'll see, I guess. I highly doubt they'll keep it around in any meaningful way. It would just be robbing sales of their newer, more expensive phone. They'll keep it listed as a current product, yes, because they don't want stock left sitting unsold on shelves that has to be deeply discounted to get rid of it.
As for development cost, the brunt of the work is borne by the HTC One. It's not a huge job to repurpose the HTC One stuff for the One X and One X+.