Symbolism, Imagery, Allegory
Status is so omnipresent in this book that it's tempting to rename it First World Problems, Plus Tuberculosis.
It's also worth discussing as a symbol, most of all because the English culture that sets the stage for The Wings of the Dove has a way different relationship to money and status than a culture like, say, the United States circa the 21st Century.
In the United States, the relationship between status and money is pretty straightforward. If you're rich, you're essentially upper-class. Done and done. But in England, it's actually possible to be upper-class and not have money. It's equally possible to have money and not be upper-class. That's because the British upper class is tied to stuff like history and family lineage just as much as it is money.
When Milly first arrives to England, for example, we find out that "Milly, it's true, […] has no natural sense of social values, doesn't in the least understand our differences or know who's who or what's what." (2.6.4.102). Don't misunderstand: Milly is stinking rich. But she doesn't have the same sense of class values that the English do. She wouldn't think twice about marrying a poor man like Merton if she truly loved him, whereas an English person would find it scandalous for someone from the upper class to marry a commoner.
Unlike Milly, poor Merton Densher is kept from marrying the woman he loves by Britain's class system. As we find out, his fiancée's aunt believes that Merton "won't ever be either a public man or a rich man. If he were public she'd be willing, as I understand, to help him; if he were rich—without being anything else—she'd do her best to swallow him." (1.4.3.33).
What this quote means is that Merton wouldn't necessarily need money to be upper class enough for Maud. But he would need to have some sort of social power; he would need to be a "public" man, like a politician or lord. Ugh. Confusing, right?
The differences between what wealth means for an American and what wealth means for an Englishman are representative—and symbolic—of the huge cultural divide between American and England. And Henry James was fascinated by this cultural divide.
In essence, while the family from Duck Dynasty might have all the money in the world, Aunt Maud would, at best, be cold and civil to them. But if a penniless lord with a gambling addiction wanted to marry Kate, Maud would probably roll out the red carpet for him.