I know it isn't said in the movie...that I can remember, but does anyone know how Peggy came to hang out with the rest of the women? She doesn't have a rich husband like the rest, or isn't rich herself. She isn't a lady who lunches, and isn't a biatch.
1) Her husband might be an up-and-coming employee with one of their rich husbands (Maybe with Mary's Stephen).
2) They met at some social event. I do get the idea that although not nearly as wealthy as Mary & Stephen, she and her husband do have some money--old money or money from his job.
Also, it might be that she was born "rich-ish," meaning that she wasn't swimming in it, but that she did receive income from some sort of annuity and/or inheritence that might have also been drawing investment income. That might have been what she meant by her "own little income."
She could have been from a old money family that was name-rich and (comparatively) cash-poor at that point. Being in love with Johnny, however, she married him instead of marrying a rich man for his money, which probably would have been an option for her.
If she could afford a car on her own, she certainly was rich. Her husband is not. She was born upper-crust, but married a working man who resents her wealthy background.