Reports have been showing that a 4 GB HBM2 stack costs $80. AMD Vega GPUs make use of 8 GB HBM2, two stacks used is $160. When compared towards GDDR5 that is expensive. 

That observation is made by Talha from segment-next, albeit I have to state here that he doesn't site any sources e.g. we cannot verify whatsoever that the pricing is correct. However, if true it is a pretty significant part of the Vega Bill, as the 160 bucks is just the two memory stacks. We wonder how expensive it'll be to make the GPU, and then the bill of materials used for all components to make an actual graphics card. 

That price however is fuctiating up and down based on market demand and lots of variables. It's clear though that GDDR5 is far less expensive to mass produce, even GDDR5X which is used in the 1080 Ti would be far less expensive to make, according to the report:

AMD Vega GPUs will use 8 GB HBM2 memory and this means that there will be two stacks used so this means that the cost of producing this will be $160. Comparing this to GDDR5, it is very expensive indeed. AMD is also using HBM2 memory for their corporate customers and the GPUs are custom made according to needed requirements of the customer. It is important to note here that the price is a rough guideline and the cost can go up and down a fair but depending upon what the customer needs. No matter how you look at it, it is clear that GDDR5 is still less expensive to mass produce and the technology needed is readily available. Even GDDR5X which is used in the 1080 Ti is less expensive to make.

AMD RX Vega HBM 2 8GB Memory Stack Reportedly Costs $160