Well for one, it's not just the price -- I can't slap a million dollar price tag on my house and call it the value -- it's the price in a well functioning, liquid market. I.e. someone has to be willing to pay it.

Historically from time to time you do get these anomalies and bubbles where things get out of wack. The tulip bubble is a classic example, and BTC last year looked a lot like that too. I think the way to connect those prices back to value is that the price eventually (and not even that long from a historical perspective) got back in sync -- tulip bulbs eventually crashed and BTC has lost 50+% of it's value in the last year.

So I guess it's not to say price always == value for everything at every moment in time, but exceptions don't come around that often. Again, the Yudkowsky book explores when they do.

Related is this comment by Warren Buffett:

"I could improve your ultimate financial welfare by giving you a ticket with only 20 slots in it so that you had 20 punches—representing all the investments that you got to make in a lifetime. And once you’d punched through the card, you couldn’t make any more investments at all.

"Under those rules, you’d really think carefully about what you did and you’d be forced to load up on what you’d really thought about. So you’d do so much better."

It's the same answer. The idea is that the free market acts out its own price-discovery mechanism and assets are valued in relation to what speculators / investors are willing to pay for them.

This is an interesting segway into auction theory and Dutch vs English auctions in particular, if you are interested: https://mikebrandlyauctioneer.wordpress.com/2012/07/29/engli...

Their argument is value is a moment to moment price and not a fundamental property.