Diamond quality can slightly vary for the same creation, depending on where and when it has been produced, so even if yours are G VS, it doesn’t mean they couldn’t use different quality/color for a different piece.
About the brand engraving - the comparison you offered is very unrealistic. If Tiffany and Costco diamonds cost the same, then either Costco is a luxury brand, or Tiffany is a mass market. The true question is how much money would I be willing to pay for Tiffany engraving on a diamond? And the answer is zero.
I’m just saying your anecdotal and hearsay evidence isn’t as compelling when mine directly contradicts it.
Anyways, yes, that is the question. How much are customers willing to pay for branding? As another example, if you had two bags with the same construction, the same material, the same craftsmanship, and the same price, but one said Gucci and the other said Target, no one would choose the Target bag over the Gucci bag, even if they were identical in all other respects besides branding. I’ve only ever bought Gucci as gifts and not for myself, but even I would not be so self-loathing as to say that I would seriously consider choosing a Target bag over a Gucci bag if the prices were the same.
The fact customers are willing to pay a price premium is reflective of brand equity. If the Gucci bag were priced just $10 more, 99% of customers would still choose Gucci. If it were $100 more, maybe 97% of customers would be willing to pay for Gucci. Eventually, you get to the point where customers have a more serious deliberation between choosing a $2000 Gucci bag and a $200 Target bag.
Thus, when you say that Tiffany jewelry costs 5 to 10 times more, that’s because of the company’s brand equity. And when you say that you wouldn’t be willing to pay ANY additional premium for a Tiffany diamond than a Costco diamond, you simply sound contrarian for the sake of being contrarian. It’s not a meaningful or authentic contribution to this discussion. And even if you yourself held a view against Tiffany because of some personal vendetta, you surely understand that you’re not being objective when you make such sweeping and unyielding statements about every product offering across every category that a company has ever made across all time.