The price level is more or less arbitrary, but the value of fiat money is not just a shared hallucination. It ultimately comes from taxes. MMTers say that Taxes Drive Money: http://neweconomicperspectives.blogspot.com/2011/07/mmp-blog...

Some anthropologists say that this is how money got started in the first place around the palaces in ancient Egypt. In more modern history, there are stark examples of how "taxes drive money" really does work to monetize an economy; search for "Hut taxes".