They are getting slightly cheaper - I am tempted to buy a 1920x1200 Dell IPS which is about £300 at the moment (still over twice as much as a regular monitor).
1920x1080 panel facilities exist because of a huge demand in televisions, and, so, the panels can be reused cheaply in computer monitors.
1920x1200 isn't really existent for consumer electronics and as 1920x1080 is "good enough" for the majority of people, so, 1920x1200 panels are a lot more expensive (due to the much higher manufacturing costs).
All this being said, it all comes down to supply and demand!