Genesis 18:27-30 meaning

Abraham speaks to God very humbly and in reverence, noting that he is but dust and ashes. However, like any good trader, Abraham endeavors to strike a better "deal." In fact, four times during their conversation Abraham shows his respect, recognizing that he is  speaking to God about the fate of Sodom. Abraham clearly recognizes he has no trading leverage. But he does know the character of God, and that is what he appeals to. If God is willing to spare the city for fifty, would He spare it for forty-five? God answers in the affirmative. 

Abraham continues. It is a reasonable progression. Abraham continues to bargain to spare Sodom in increments of five. The Lord's response is always affirmative each time Abraham offers a successive proposal: Suppose the fifty righteous are lacking five? Suppose forty are found there?" When Abraham gets to forty, he shifts to increments of ten. Suppose thirty are found there? With each question, God assures Abraham that he will not destroy the city if that many righteous people are found. 

Abraham shows humility and reverence toward God. Twice he asks that God not be angry with him (verses 30, 32). He humbly describes himself as unworthy to question God so directly (verse 27). Of course God knows how many are righteous. But He does not reveal that to Abraham. He allows Abraham to intercede, and play a role in the outcome of Sodom. By involving Abraham, God is teaching him and providing a lesson he can pass along to his descendants.