Hmmm, I disagree with that to some extent.
A 5yo whose owner spent $5k to put him in training for 60 days with a great young horse Dressage trainer, is inherently worth more than a 5yo who had 60 days of reasonably good amateur training put into him.
At some point there is a point of diminishing returns though, so the 10yo going around AA Hunter courses and winning, isn’t inherently worth more, or less than the “same” 8yo who has less $ in him, or the “same” 12yo who has more $ in him.
A 4yo lightly backed horse who has been ponied on trails all over, for 2 years, is worth more than a 4yo lightly backed horse who hasn’t ever left the property he was born on.
The level of training (time and/or $ cost) does influence how much a horse is worth.
The level of exposure (time and/or $ cost) does influence how much a horse is worth.