The ".es plural" form of a domain hack is, in my opinion, almost totally made up in domainers' heads -- ever since del.icio.us came into existence, the domaining world's view of what constitutes a viable hack has expanded and expanded.
Additionally, speaking from experience in the gold and metals niche, no one really says "gold prices." Even when discussing multiple price points, say over the course of a five-year chart, the singular is generally the term used, eg. "the gold price over the last five years." This is mainly because the main gold price, the London Fix, is the "universal" gold price, and even though relatively little gold actually gets bought or sold at that price, it's the standard beginning point for all bargaining/arguing about the final price of a sale.
So ... an arguably specious hack, of a technically incorrect phrase ... personally, hard to see any market value. You could probably flip it to another domainer for a minor profit though; true believers in the ".es plural" hack will likely buy it just to prove that someone will.
Frank