There's a lot of different figures floating around, but a reasonable number to throw out for the average income in, say, 1939, would be $1500 to $2000 a year. That was enough to take care of a typical American family by reasonable standards of the time -- rented house, inexpensive car, a radio, maybe a few modern appliances. There wouldn't be many luxuries on this wage, though, so the Average Wage-Earner wouldn't be likely to own an especially fancy pen. Those dollar Esterbrooks from the drugstore would be more common.
Somebody truly "middle-class" by the standards of the time -- owning a home, driving a La Salle or a Chrysler instead of a Chevy or a Plymouth, having at least one servant, sending the kids away for the summer so mom and dad could travel -- would need a good bit more money than the Average Family. But you could live extremely well on, say, $5000 a year.