Quote from peilthetraveler:
The answer to the price of milk has to do with supply & demand. In 1999 the production of milk in the US was around 70 million tonnes. In 2010 the production was 87 million tonnes. Thats almost a 25% increase in production, yet the population only grew about 10% in the same time period. So what happens when you have more supply than demand? The price goes down (or stays the same). Now if milk production only increased 10% instead of 25%, I think we would be seeing much higher milk prices today.
But that's the nature of productive assets. They grow. Gold never grows. An ounce of gold is always an ounce of gold. Even if the ounce of gold appreciates, it doesn't create more gold. Productive assets do. In the past, the vast majority of wealth was in land. That's not true today. All the gold in the world is worth $10 trillion and in the long term is shrinking as a proportion of total wealth. The value of total US assets is around $200 trillion, and growing.
http://rutledgecapital.com/2009/05/24/total-assets-of-the-us-economy-188-trillion-134xgdp/