Who is the most expensive transfer in FC Porto?

Can you tell me who holds the title of the most expensive transfer in the illustrious history of FC Porto? I'm fascinated to learn about the player who commanded the highest fee when making the move to this esteemed Portuguese club. Was it a superstar striker who lit up the pitch with his goals, or perhaps a talented midfielder who orchestrated the team's play from the center of the park? Either way, I'm eager to uncover the identity of this highly-valued acquisition and understand the impact they had on the club.