Just my two cents... I was always under the impression that copper was a commodity and price was driven by the market? I don't know HD could get a better price then a supply house weather they bought volume or not.
Sales volume is different for Home Depot and Lowes when compared to a supply house- they may be able to mass negotiate as a corporate giant, but individual stores don't necessarily sell that much. When prices rise and the big box is still cheap, they're quickly bought out. But when the prices drop, the big box often lags behind... until they're left with 250' reels of romex and 2" copper pipes running twice what it's going for at the supply house, and it just sits there for weeks or months until some poor unknowing DIY sap buys it.
