Yeah, but what if....
"That would mean those metals would be quite a bit less precious once mined"
No, it's not metal.
Ceres is a world made of a wonderful, frozen alcoholic beverage. Something along the lines of 'vega vodka', 'betelguise brandy' or perhaps 'alpha centauri calvados'. It was placed there by an ancient, technologically advanced race of distillers. These distillers, realizing that humankind would probs get bored of space travel after playing golf on the moon, wanted to give us something else to shoot for. To paraphrase Kennedy 'We choose to go to ceres, cos there's a load of cheap booze there. Think of it as the Lidl of the asteroid belt'.