At the funeral of one of their own, the Others dress in white, with shirts that are similar to Jacob's.
Mr. Eko, waking from his fever, sees Yemi, who tells him it is time to confess, and be judged.
Wandering through the jungle later, we see a flash of the black smoke in the background shortly before a number of figures - people who Eko caused the deaths of - appear, the last one, his altar boy, telling him again to confess.
When Eko stops to drink at a river, the smoke rears up behind him... then flees when he turns to face it. John Locke emerges from the woods moments later, followed by the whole group from the camp.
Locke tells Eko that what he saw (back in season 1, in Walkabout) was a very bright light, "it was beautiful". Seems he saw something very different from Eko's black smoke.
Yemi's body has vanished from the plane.
Yemi appears and leads Eko into the jungle. He asks if Eko is ready, and is answered yes. When Eko holds out the cross, Yemi touches it. Eko explains that he does not want forgiveness and does not feel that he has sinned, only done what was necessary to survive. Yemi looks angry. "You speak to me as if I were your brother." Eko, who knows now that it isn't Yemi, chases after him. The black smoke appears, and beats Eko.
Before he dies, he tells Locke something. Locke relays the message to Sayid as "we're next".
So, to recap: The Cost of Living is the first episode in which we see the smoke in its role as judge. It is also - unless you count the visions of Christian in the Pilot - the first time we see the monster take the form of a dead person (all previous visions were in dreams or hallucinations). I'm surprised the smoke didn't get credited as a guest star, it was so prominent in this episode.
What can we tell from its behaviour here? As soon as he learned that Eko did not feel as though he required forgiveness, he stopped pretending to be his brother. So repentance may have some importance here. Perhaps this is just his disdain for the people who bring destruction to the island showing, though.
He did not drop the form of Yemi until he was out of sight, which seems to be a pattern. and of course we see Yemi touching objects but not people. It almost suggests that the only contact he can have with normal humans is while in the form of the black smoke (perhaps Richard's condition makes him an exception?).
Josh Holloway was excellent, I thought. Sawyer as a character and Josh Holloway as an actor when you look back from the very first Lost episode compared to this week's episode and you can't help but to notice the difference. With Kate, there's no difference. No difference in the actress. No difference in the character. 
