My understanding of the ending is that Patrick Bateman is in search of a resolution to how he feels. After all of the murders and chaos he creates, he feels a moment of catharsis and then indifference. He has gained nothing and he will never be able fill this void.
Throughout the film he can never really obtain the recognition from others that he craves. He is almost indistinguishable from his colleagues and people forget his name. Even when there is a mountain of evidence against him for the murders and he confesses to his lawyer, it is written off as a joke. Still, nobody sees him, and not even for the psychopath he is. And nobody cares because they're wrapped up in their own lives.
And finally, Patrick Bateman is hollow. He lacks emotional depth and substance as a person. That's why he goes into these extremely boring and long monologues about skin care and business cards.
There is an idea of a Patrick Bateman; some kind of abstraction. But there is no real me: only an entity, something illusory. And though I can hide my cold gaze, and you can shake my hand and feel flesh gripping yours and maybe you can even sense our lifestyles are probably comparable... I simply am not there.
So, in the end Patrick can not escape his hellish existence. A lot more of the movie can be interpreted as a criticism of American values and society but that's a different discussion.
