It takes both the buyer and the seller to set the price. Yes, CP3 could have just walked a few days later, which lowered his trade value. However, it is still possible that Houston could have gotten CP3 as a free agent. The fact that they gave up what they did does not represent the value of CP3 in a trade, it represents the value difference between the likelihood of acquiring CP3 in a trade and the likelihood of signing him as a free agent and the difference between benefits of each scenario.
The value for Paul George will not just be set by the Pacers, but by whoever trades for him. That means the Celtics are part of the pricing mechanism.