Agüero will not go for anything less than €60 million which is his buy-out clause.
Atlético's board don't intend to sell him this summer and negotiating a lower price isn't possible. The club plays a 4-4-2 traditionally with wingers which means they play without a proper playmaker, a #10, like Diego is. Besides, the club also signed Fran Mérida from Arsenal and he can play in that position if needed while Jurado is a #10 as well, so I don't see a reason to have three players of the same characteristics. Quique though is using a 4-2-3-1 at times, often away from home, but still no reason to get Diego because of the reasons listed above.
Also, regarding Kun, he's got a extension on the table; his current contract is running out in 2012 and the board wants to extend to 2015. He'd get an improved salary and of course, the buy-out clause would be raised to €80-90 million which would make it even harder to buy him. He'll most likely sign it after the World Cup but Atléti will sell him, either this year if someone pays the buy out clause or next season for something ridicolous.
Regarding Tiago, well, Atléti want a new loan as they won't pay €8 million, though Juventus have reason because he's been great during the second part of the season, surprised perhaps? Well, that's the truth.
Cheers.