Longenecker emphasizes that the following figures are only rough estimates. But it seems that he has done extensive research on this and there are good reasons to use them as fairly reliable background information when it comes to interpreting Paul's letters in the New Testament.
Here is what he says,
"1. Roughly 10% of this community is among the middling groups of Roman urbanism (ES4), not without economic risk, but also with a relatively significant level of economic security...
2. Roughly 25% of this community has some minimal economic resources (ES5)...
3. Roughly 65% of this community is marked out by subsistence-level existence (ES6 to ES7)."
It is clear that the majority of the Christ-community in Paul's house churches lived below, at, or near subsistence level. This, I think, means that we should read Paul's letters in light of this important background.
For example, when we see the words "suffering" or "affliction" in Paul's letters, at least one aspect of these words would be related to socioeconomic hardships from the audience's perspective (unless the context clearly says otherwise).
Also, financial generosity in Paul is not so much about the wealthy giving to the destitute. Rather, it is about sharing the scarce resources available in the community. That is, it is about the relatively "less poor" Christians sharing their resource with the poorest among them.