If Zen 100% reaches Haswell's IPC - and clockspeeds - we can expect a quad core APU with half of RX 460's graphics performance to fetch only about ~$150 without SMT, maybe $200 with it (depending on its performance).
If AMD were to add HBM - a single stack would add ~$40 to the total cost... effectively without regard to capacity.
* Enlarged die to add HBM controller(s)
* Additional interposer (+handling)
* Custom APU package (to interface interposer to socket)
* HBM stack itself
Due to the need to simplify design and manufacturing, AMD would only use one die for all APUs - so every APU they make would be enlarged by the size of an unused HBM memory controller... just so they can have graphics performance only 33% worse than the RX 460 instead of 50% worse...
There just isn't a business case for it... on the desktop. A 'massive' integrated GPU with HBM, and multiple 8-core dies makes perfect sense for the server or HPC markets - and could more easily justify its development costs... once you have gone MCM, using an interposer might just make things easier overall, permitting a higher-performance inter-die communication bus.
The thing is people only focus on the APU but dont see the rest of the Laptop.
If the APU has 50% LESS performance than the one with HBM2, that means the APU with HBM2 memory is 2X faster (1).
If you want to make a Laptop with 2X the graphics performance of ZEN + DDR-4 3200MHz, you will need the APU + a dGPU.
That will increase the BOM price because of the dGPU cost + GDDR-5 cost + Cooler (2), increase the power consumption (3), increase the heat output of the Laptop, increase the height because of the bulkier CPU+dGPU cooler (4), increase the BOM and complexity of the Laptop motherboard to facilitate the extra dGPU (5), decrease the Battery life(6).
Now tell me that if you were a Laptop manufacturer you wouldn't want to have an APU + HBM2 that will offer you all those above (1 through 6) at the same or lower cost + lower consumption + the ability to have smaller Battery + the ability to make thinner design ???
The problem for AMD is not the higher cost of the HBM2, the problem is to find a customer for that APU. AMD doesnt have the luxury to create this APU and wait for customers to use it, they need to have the deal before they will make the product due to their current financial state.
Also, i dont know how much more extra space the HBM2 controller will add to the die, in the past AMD APUs had dual memory controllers DDR-3 + GDDR-5 and DDR-3 + DDR-4. They could very well add another controller to the ZEN RR die and be ready to use HBM2 dies when the customer wants them or HBM2 price is lower for the mainstream market. Pure speculation on that part but it could be doable as 14nm FF will allow this and save them the cost of extra die design/masks etc etc.
Edit:
Also, SAMSUNG recently announced 8GB of HBM2 modules to be available by the end of the year if im not mistake.
A Dual Core + HT 4x Threads with 8x CU Polaris iGPU + 8GB HBM2 memory for both graphics and System memory at 4.5W TDP would be a killer product for Microsoft tablet based products like Surface 4 Pro. The will have exceptional high graphics performance for games AND professional 3D/ Open CL workloads and acceptable CPU performance with 8GB of HBM2 memory that will save lots of system space, complexity, BOM, decrease heat output, allow thinner cases etc etc.
Or a Quad Core ZEN APU with 8GB HBM2 for Laptops could easily allow the same savings by completely eliminating the usage of DDR-4 memory.