HBM2 isn't that expensive. Interposers are pretty cheap, it's not what's significantly adding to the cost of an HBM solution. Interposer can be done on a 90nm process or similar (watch AMD's presentation on it). What adds the cost to an HBM solution is the extra tooling to do the packaging, and the special memory required.. and there are only a handful of packaging places that can do the work. The additional steps in the process of putting dies on interposer also lower yields, because each step adds to the failure rate. Part of the reason AMD offers a free license for HBM2 is so that they can lower the cost of manufacturing. Packaging and economies of scale from SK Hynix.
Why I mentioned Apple is because Apple more than any other manufacturer would be willing to pay the cost of the difference for the benefits HBM2 offers. Lower power, better bandwidth, and the biggest advantage to Apple in their pursuit of ever thinner devices the space savings.
Take this mobile Intel CPU:
http://ark.intel.com/products/93336/Intel-Core-i7-6970HQ-Processor-8M-Cache-up-to-3_70-GHz with its $620 pricetag. An HBM2 Zen based APU could be built for that price range while providing the cost benefit of simplified motherboard (no need for the memory bus and PCB traces) and integrated HBM2 memory.
I would argue that AMD could build HBM2 based APUs and still have better margins than they do on HBM2 GPUs.
Not to mention this same product could have applications in high density blade servers. And other benefits like iGPU not being memory bandwidth starved.