Ralph Moody's books, beginning with Little Britches. Kind of like the Little House books, but centered on a boy and ranching out west. "Eggs is eighteen cents a dozen!" became a family saying after my husband read these out loud to the kids.
<i>Five Little Peppers and How they Grew</i>
Sidney Taylor's <i>All-of-a-Kind Family</i> books. NYC's Lower East Side in the early 20th century. The central family is actually better off than many of their neighbours--they have a whole flat instead of a tenement--but the general environment is one in which everyone has to be careful with money.
Many of Joan Aiken's children's book, the <i>Wolves</i> Chronicles and others, feature plucky, intrepid hero(in)es whose poverty is one of their sources of strength.