The Spaniard joined the Magpies from Tenerife in 2014 for a measly fee of less than £3 million.
He spent five seasons at the Tyneside club and enjoyed a brilliant 2018/19 campaign under Rafael Benitez where he scored 13 goals in all competitions.
However, he was sold for £30million this summer, after Leicester triggered his release clause.
Although Newcastle made a huge profit on Perez, Ashley feels that he has made an error in this case.
Considering how inflated the transfer market is at the moment, Newcastle should have sold him at a much higher price had there been no release clause or a higher buy-out figure.
Ashley is adamant that he has learnt his lesson from this deal and has revealed that the Magpies will put punitive buy-out clauses in all future player contracts.
“We didn’t want to sell Perez, but had no option once Leicester reached that figure,” said Ashley.
“These clauses are the highest you think the player will accept, so he can’t batter your door down and demand to be sold for £10m. I don’t care what people say, it’s very difficult to keep a player when they want to go. The age of slavery is dead. But we’ve learned our lesson on release clauses from here.”