I actually think it was easier for Milan to get more. First because of all "Kaka world acknoledge" crap, and secondly Kaka didn't want to go. Milan didn't exactly told that they want to sell Kaka, so they played it cool, like nothing will happen unless you will give us "right bucks".
With Inter, Zlatan wanted to leave, not the team wanted to sell him, or neighter, and when player wants to leave the team always suffers in terms of economics. Look at Arnautovic, will be the best example. Twente right now has close to none in saying. Arnautovic would never be treated at Inter, if he didn't want to come to Inter so bad.