I dug a little about HBM2. Low-Cost HBM2 was all related to only one thing: reducing the number of TSV's, so Yotsugi was right here.
The other side of this coin. SK-Hynix has announced HBM2E, which have up to 460 GB/s bandwidth from 1024 bit memory bus. From what I can understand, you can have 512 bit memory bus, by simply lowering the amount of TSV's, which can work both ways: you can lower the bandwidth, and complexity of the build, and the cost related to it, while at the same time increasing the bandwidth available for the GPU.
512 bit, 4 GB memory stack could, in theory, offer 230 GB/s while at the same time offering reasonable manufacturaing costs for even low-cost products.