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Having RAM on both sides does cost more money, though. The question is, how much more? $5, $10, or $25? Probably closer to $5, but I don't have any inside sources to tell me precise amounts. That approach (dual sided RAM) used to be pretty common ten years ago, but potentially the tolerances for GDDR6 make it more costly today.
The extra 8GB of GDDR6 meanwhile probably costs about $25. So if BOM goes up $30, having retail price increase by double that amount is pretty typical.
But even if BOM is $50 higher, Nvidia has margins where it could eat that cost and offer the 4060 Ti 16GB at a $50 premium. Because the 4060 Ti really should have been a $300 part to begin with.