Because this isn't going to inflate the unit cost by hundreds. They could probably sell this at like $400 for a reasonable loss here.. I mean the GPU and CPU are good, but they're not some crazy home brewed creation that them millions- like Cell. They're fairly standard parts courtesy of their relationship with AMD. The thing is an APU.. An AMD APU.
So, to me, the most expensive aspect will be the memory yes. But it won't drive it up to PS3 cost levels.
For that matter, GDDR5 while expensive is still not as exotic as the XDR they shipped with PS3 attached to Cell.
PS3's "high cost" components:
XDR
Cell
Blu-Ray drive
PS4's "high cost" components:
8GB of GDDR5
Because:
Blu-Ray drives are far, far cheaper.
They're not using a complex custom CPU or GPU.
They're not using highly complex and niche memory, like Rambus XDR.