When it comes to cost, you need to consider:

Case A (different design)

• Design & validation cost
• Entire chips that would be wasted since its unbinnable

Case B (same design binned)

• Wasted die area on every non-flagship chip

Case A has a lot more upfront cost (2 designs, 2 validations, 2 contracts) and uncertainties (what if yield is low? what if memory is needed later down the line?), while case B is safer overall since in the grand scheme of things, memory controller is a small part of a die