It's not really $500,000 though is it? As soon as you exchange a small amount of that into dollars you crash the exchange and the value drops significantly. I'd estimate it at about $30,000 worth.
That said, I think you're overstating the impact this would have. The recent downswing from ~$30 to ~$10 in a period of a couple days was speculated to be due to MtGox consolidating 20 times this amount.