Interesting question posed. As a owner of both these brands, the quality of GS finishing is obvious. I would tend to think that the margins are extraordinary in luxury products and I'm sure Rolex's are significant - is there a way to find out by looking at annual reports of other listed luxury brands etc? I know in fashion, the cost price of manufacturing clothes are less than 10%. As for Seiko manufacturing costs themselves, the fact they can make decent watches for sale at hundreds of pounds seems to confirm the margins are extraordinary.
Why they charge less than Rolex is probably a lot to do with marketing. Looking at the amount of Rolex advertising in magazines, sports events suggest they spend an enormous amount on marketing compared to that of Seiko / Grand Seiko. Maybe this will change but I'd have thought Rolex price their products to reflect this.
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