The thing is, you're not going to mine 500 lbs of platinum, it's quite possible you're going to mine millions of pounds of it, along with other valuable metals. Yes, you wouldn't be able to dump that on the market too fast, but you would put all the existing platinum mines out of business quickly. And with lower prices for platinum, more uses would be found for it, increasing the demand. More people would want it for jewelry and aesthetics probably, but also it's quite likely new industrial uses would be found for it which were previously unexplored due to its extremely high cost. You have to think more long-term about these things, which it doesn't look like you're doing with your analysis. An endeavor like this isn't going to be something small, it's going to be absolutely huge, and mining a single asteroid will span for decades most likely.
Finally, look at the environmental aspect: mining is terrible ecologically. Wouldn't it be better to do as much mining in space as we can, so we aren't digging giant holes in the ground, polluting groundwater and rivers, shearing the tops off mountains, etc.? We just had a bad incident with river pollution in one of the western states (CO I think), and mining always has problems with environmental opposition in advanced nations (and in backwards nations causes all kinds of problems, like fueling conflicts as with coltan). Environmentalists won't care if you break up asteroids for mineral resources.