If a 1.5 GB Graphics card performs identically to a 3GB Graphics card, why is the 3GB $200 more expensive?

I saw a 1.5GB GTX 580 and a 3GB GTX 580 for sale. The 3GB card was $200 more expensive. I don't understand this when all benchmarks show that the two cards have almost identical performance in games, which included games like crysis on all max settings and metro 2033. In some cases the 1.5GB card even... show more I saw a 1.5GB GTX 580 and a 3GB GTX 580 for sale. The 3GB card was $200 more expensive. I don't understand this when all benchmarks show that the two cards have almost identical performance in games, which included games like crysis on all max settings and metro 2033. In some cases the 1.5GB card even outperformed the 3GB card. I then assumed that more memory on a graphics card means absolutely nothing performance wise, since even at the highest resolutions and with multiple monitors the benchmarks of the two cards were still identical. So why is the 3GB card so much more expensive?
Update: After looking at the first two answers, I now understand why, I think it's because it's a marketing scheme for idiots(like the first two answerers) who think that more memory on a GPU gives better performance or they think it means they are getting two cards in SLI.
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