Update: There's a short article in ZDNet Korea in which LG Display's CEO says that OLED TVs will be competitive with LCDs in 2014. LG Display will only mass produce at that time. So it's not clear if LG is indeed planning the 5.5-Gen plant for OLED TVs or smaller displays.
LG didn't disclose how much money it will invest in this new plant. But Samsung's own 5.5-Gen 3-lines AMOLED plant (planned for 2012 as well) will cost $2.2 billion.