Would be interesting, but you have to account for the fact that they usually do not own much of those players, 3rd party investement funds buy most of the rights of the players and then cash big time with it. Without those 3rd parties, Portuguese football would've been nowhere near the level they are now.
You have missed an important part: at the time of sale Porto owns 95 or 100% of the players.
Example: Hulk. Bought from Japan 5.5M for 50%. 2 years later the club bought the remaining 50% for 8M. That deal was done at the time of the purchase. And then he was sold for 55.
Same goes for James Rodriguez etc.
Th 3rd party is used to have cash flow in some occasions.
In recent years Porto have bought some expensive players for Portugal finantial reality: Danilo, Jackson, Alex Sandro, Quintero, Herrera and Reyes. Danilo was bought with BMG, a Brazilian bank as a partner. But he was way overpriced....terrible deal.
It´s not the perfect deal but he has been working good in the the last 20 years.