Today FC Porto confirmed Jackson Martínez’s transfer to Atlético Madrid for a fee of € 35,000,000, the 5th highest transfer fee they have ever received. Having bought him for € 8,8M from Jaguares in 2012 they made a profit of a bit over 26M on the the Colombian. There were 6 transfers deals that yielded FC Porto an even higher profit*. Some stats from the 25 highest transfer fees that FC Porto received since 2000:
Per season data:
*Obviously those are not direct profits as a large share of the transfer sums will go to third parties. In the case of Porto a lot of players are bought with help of investments funds who later claim (part of the) ownership of those players.
Note: all data comes from transfermarkt.de