Disclaimer: I don't own Naruto (but I'm working on it).
The price to keep the boy dormant at the back of his mind was simple. He scoffs at it even now.
In the Akatsuki base in Rain, there is a room protected by everything from locked doors to genjutsu. It is laid out like a typical hospital room. The medics who are allowed in are very carefully chosen. Kabuto was nearly one of them before Orochimaru freed him from Sasori's control. Itachi suspects the contents of the room have something to do with whose body Madara is using but has been warned off even going near it.
The "patient" (there is nothing wrong with her) is a brown-haired woman with purple stripes on her cheeks. Her name is Rin and she has been sleeping for fifteen years. In the chaos of the Kyuubi's attack, it was very easy to spirit her away and she has been in there ever since.
When he is not on missions, Hatake Kakashi visits the memorial every morning. Hers is one of the names carved on it. Madara wonders what would happen if Kakashi ever learnt about the room.
The second part didn't come into play until Akatsuki actually began collecting the bijuu. As far as he knows, Kakashi doesn't realise the real reason he has survived all his encounters with Akatsuki so far. It was a very subtle suggestion on his part because the other members, Itachi in particular, would be far too interested in the fact that Kakashi's continued existence is part of the price he must pay in order to ensure that his body remains under his control.
This is related to his worst fear. He is afraid that, any day, Obito will decide that their deal is not enough and forcibly take back control of the body. Neither of them would emerge from the subsequent struggle unscathed but despite the fact he has spent fifteen years learning about this body, Obito was the one born into it and that counts for more. Also, for all his power, his greatest enemy is his own body and that is what truly scares him.
The last part is the easiest. In the guise of Tobi, he allows Obito very near the surface of his mind and uses his suggestions as how to behave, especially around Deidara. Sometimes, he thinks "Tobi" has taken on a life of his own. He does not mourn Deidara as his death allows him to shed the Tobi persona. Obito does because he reminded him of Kakashi and what he could have been.
No line breaks needed!
Credits: the comparison between Deidara and Kakashi was inspired by Shtuff's Seasons of the Heart. The ideas about Tobi's personality were also a result of it.
Did you know that Obito has the same birthday as Madara's brother? Coincidence? I think not.