Actually, Karim, a Trade Exception is NOT a cost. Not directly, anyhow. If I send you a player worth $16mil, and you send me a player worth $6mil (which would require you to be under the cap), I would get a $10mil Trade Exception. You would have no say whatsoever in it, it would be automatic because the salaries do not match up (which is why you have to be under the cap).
That being said, it is a cost indirectly in two ways: 1) You need to give me SOMETHING back in return (it could be a pick... I don't know about cash though); 2) It would help improve the other team, which is mainly relevant if they are in the same conference as you (unless you expect to meet them in the finals).