Explain Mello's sacrifice to me

Also, keep in mind that Mello left Takada's box on a table somewhere, and she manages to break the window of the van she's in, get access to a cellphone somehow, and somehow burn herself and her belongings. So, Mello probably wanted to give his own life for this... It's very confusing.

Okay, after thinking about it, I have a theory which could be the explanation:

As we see in the final episode, Lidner had talked to Mello on the phone, probably told him about their plan in order to get his expert opinion on it, and heard Mello say "Only I can do this." in a defeated voice.

So Mello, during this conversation, figured that the notebook in the gym might be fake, and planned the kidnapping plot by his own initiative. In Mello's mind, if Takada dies, Mikami will continue the killings with the real notebook, if there is such a thing, and SPK will follow him and see it.

The reason why he's so sad might be because he realizes the plan would be risky for him and Matt, and/or he might be depressed because Near will be the one to capture Kira physically, but I don't think he plans on dying, because I can see no reason why that would be necessary.

He makes sure that Takada can get to her stuff in the box outside the van (so she can call her accomplices), and he's checked that box for hidden notes, but he didn't think of the possibility of her conceiling a note while undressing, which is why he doesn't mind taking off his helmet.

In the conversation with Lidner, Mello also told her not to tell Near about this plan, just in case Near would try to interfere because it's "unethical" or *beep* it up or try to save Mello or whatever. This is why the possibility of a fake notebook doesn't enter Near's mind until after Mikami goes to the bank.