I don't believe that.
Dead people cannot be more valuable.
However, we take things we see regularly for granted. That's a fact.
But a one-time offer is usually held in high regard.
 
So it's not that people become more valuable when they die. It's more like we regret the value that they may or may not have imparted if they hadn't died. We regret the unpredictable, unrealised potential that they might have had and will never realize. They could have gone on to do great things or nothing at all. And now, we'll never know.
 
To Matthew Perry, a great life worth remembering, and Chandler Bing, an even greater character never to be forgotten. Rest in peace.