I don’t know where they come up with the half a million dollars figure. At 177 ounces it’s worth about half that. Even if you take into account that they may be talking Australian dollars (about 1.05 USD) they’re still way off.
Jeez, just read that part of the article. Never mind.
A nugget, especially one so rare, will bring much more at auction than the current spot price of the gold.