Well you can get a good minimum estimate by looking at the per Gb cost of GDDR6x and simply adding that value on, there will be other costs associated, possibly with a slight redesign of the PCB and the fact you'll need changes to manufacturing process for all these things. An extra 10Gb of GDDR6 is about $117 and more expensive again for 6x I'd reckon, I can't find prices for that. I think £150 for just the extra vRAM is a good bet.
Are people going to buy that when the only examples we have of games going over 10Gb usage are also examples where the frame rates are unplayable? I dunno. I'd be interested to find out, I personally wouldn't waste 150 quid or more on vRAM I can't use.