Aribo cost over a million to sign on agent and sign on fees for the player. Stuart Robertson mentioned this during an interview.
The thing people forget with bosman deals is that without a transfer fee the players side are in a good position to negotiate so can ask for higher than normal wages and fees.
Sakala is a great example of a player trading model, sign a player for a modest wage and sign on, get a good couple of years out of him then move him on for a few million. Not every good trade has to be a Bassey or Kamara.