AgentP
Thinks mods influence posters politics. Promoted to QAnon Editor.
Here is an "old" chart. I am seeing the same price as you are. Keep in mind that the GDDR5 clock speed affects the price as well. Give that PS4 is alleged to have dropped their target from 192 to 176GB/sec (down clocked) I would align my guess with $15/GB and it may even be cheaper with contract of bulk purchase.
I would guess Sony is paying no more than $40 for 4GB GDDR5, not including cooling and integration. MS could get 8GB DDR3 for the same, they are also pushing for (currently) high speed stuff if the reported DDR-2133 is being used.
Of course the DDR3 will be replaced with DDR4 and the price will drop quickly. But maybe Sony has a plan to replace the GDDR5 with something with comparable bandwidth in the future.