You're absolutely right. Higher memory speeds place greater requirements on memory controller designs and PCB layouts to get things nice and happy with doubled up RAM modules. This can certainly cause a rise in costs for those parts. It will certainly be interesting to see if the PCBs happen to be exactly the same between the 8GB and 16GB models.Higher speed VRAM makes it more difficult to do stuff like clamshell memory designs. The fact is that since the GTX 600-series, there have been very few GPUs with memory on both sides of the PCB. Off hand, I couldn't even name the last time we had an AMD consumer graphics card that had VRAM on both sides.
We'll just have to wait and see if AMD does any "clamshell" offerings I guess. Because we don't know for sure exactly how much it increases the BOM. But like I said, a $50 increase in BOM translates to $100 retail in general. Was it a $50 increase, or only a $30 increase? I couldn't say. But there's also market differentiation and economics, so if Nvidia thinks it can charge $100 extra and people will pay it, the smart business decision is to charge $100 extra.
Keep in mind that the double VRAM will probably be most beneficial for AI workloads. There are things that you simply can't do in AI with less VRAM. Several tools (Whisper and Text Generation) end up VRAM and CPU constrained rather than GPU constrained. There will absolutely be some AI people that want more than 8GB but don't want to pay the $1200 for a 16GB 4080 or $1600 for a 24GB 4090 — especially if they can make do with a $500 4060 Ti 16GB.
There are many, very smart people working at Nvidia. I've no doubt a lot of thought went into the pricing, segmentation, and positioning of each product! For example, having two 128-bit dies releasing side by side with such small differences in overall size shows that costs have shifted greatly. No way could Nvidia or AMD justify all the expense and man-hours to tape out another die on the same architecture just to save ~30mm² back in the 28nm days. The decision to go with this rather than a 160-bit AD106 (or 96-bit AD107) shows that TSMC's 4nm process must be extraordinarily expensive.
The greater VRAM model will definitely be most useful for AI and rendering use. The recent bout of games falling on their face with 8/10GB cards is largely an issue of developers just not doing a good enough job optimizing for those capacities. Still, having at least 12GB of VRAM affords the gamer with the chance of side-stepping the issue since that's about the same amount of usable VRAM available in the PS5(the primary development target for modern games).