I agree with $500,000 prediction being too high, even $100,000 is still to high. Gold is a 5000 year asset, no way Bitcoin will surpass it in 13 years from now, just not possible! Sorry, but it's nonsense.
You could check the speculation thread and news about the bitcoin. Adoption rate increasing in sky rocket speed. Therefore, we will see the price value as 10000$ with in 5 years. I agree that Gold is traditional and long term asset, but it being use a ornament but bitcoin is a asset, money and payment mode. None of the thing was in history which have features like bitcoins.