I've made some spendy Costco runs. It's like Target, you can't just buy what you came in there for. I can however say, I've never bought jewelry at Costco, let alone a $400,000 ring. Even though I haven't, AV Club says someone has:
You may not have known that Costco sells diamond rings, but a quick search on its website shows that it actually has more than 400 such options available. Why not, it has everything else, like Clorox wipes, pajamas, and $1.50 hot dogs. But CNBC reports that someone reportedly purchased the store’s highest priced ring—a 10-carat diamond platinum solitaire—for $419,999.99 (really, you couldn’t just round it up to $420,000 at that point?).