At first glance it might seem like Nick is talking about money, but when Nick refers to “fundamental decencies” here, he is actually talking about moral conduct. He has to remind himself that people have different upbringings and therefore different perspectives on the world and ways of acting in it.
If he was to forget such a truth, he might be inclined to judge people too harshly for their actions. Recall how he says that “reserving judgments is a matter of infinite hope.” If he does judge people too soon into knowing them, then he might miss out on understanding that person on an in-depth level and overlook important aspects of who they are. For example, Nick judges Tom and Daisy, but he also gets deeply involved in their life and caught up in their society. He also judges Gatsby at first but comes to understand where Gatsby came from and why he acts the way he does. The more time he spends with this group of people, the more he understands how their upbringings impacted the way they treat one another. However, Nick admits that he can't reserve judgements forever. The fallout of the Buchanans' selfishness ultimately drive him away. Recall that he goes on to say:
And, after boasting this way of my tolerance, I come to the admission that it has a limit. Conduct may be founded on the hard rock or the wet marshes, but after a certain point I don't care what it's founded on. When I came back from the East last autumn I felt that I wanted the world to be in uniform and at a sort of moral attention forever.