In one sense yes — if the sum total of the market is '1' and everyone is just some fraction of that. Then there can only ever be relative price changes. But th

In one sense yes — if the sum total of the market is '1' and everyone is just some fraction of that. Then there can only ever be relative price changes. But the sum of all prices does in fact vary. I'm sure you're familiar with Tulip Mania. Prices can be bid ever-higher in a cycle, and they can also decline in tandem as in the depression. For the entire market to have an expanding total price, new money has to be entering the market, or the net real value of the underlying financial assets has to be declining. Either way, the entire market can become overvalued or undervalued, especially compared to non-financial (or illiquid) assets denominated in the same currency — e.g., wages, energy, or land.