That's because memory channels cost money. Memory controllers are more complex. Lastly, chips that can make use of high bandwidth VRAM are both of the above.

Memory chips are a commodity, that I agree. Though HBM is trending towards not being a commodity.

Memory controllers are die area. mm2 die space is linear bom cost.

DRAM+mm2 bom will have a different slope to just DRAM bom but still basically linear. Nonlinear pricing is pure market segmentation.

  Memory controllers are die area. mm2 die space is linear bom cost.

Ignoring the design and platform support that comes with higher bandwidth memory controllers.

And this is not also linear or fixed cost, why?

I don't think you know how industry pricing works. Wafers have a price, double mm2, double the price of chip in bom.

Yes. Bigger die size. More complex memory controller designs. More memory lanes. More software support for higher memory lanes. All adds up.

I'm explaining why despite memory being a commodity, high memory bandwidth VRAM cost is not cheap.

Memory controllers are copy paste. It's not more complex.

It's more expensive in a linear way wrt bom.