According to IHS, Samsung's Galaxy S5 costs about $256 to make, and the most expensive component is the 5.1" FHD Super AMOLED display which costs $63. That's actually cheaper than the 4.99" display on the GS4 which was estimated at $75 back in 2013 by IHS. In 2013, DisplaySearch estimated the GS4 display at just over $60.
According to reports from Korea towards the end of 2013, Samsung Electronics indeed asked Samsung Display to reduce its AMOLED prices in an effort to reduce smartphone costs (even though IHS esimates that the GS5 costs about $20 more than the GS4, even with the cheaper display). Then again it may simply be that DisplaySearch were correct and the $60 price remained the same even though the new display is a bit larger and performs a lot better.