Don't worry, Glenn: Welles loved digressions! :;):
I just remembered now that the Ambersons figure I saw was in, I believe, the Jan. 1943 issue of Variety; it may have been the first year they published box office receipts for the previous year. Each studio had a list of their movies which had made one million or more at the box office in 1942: Ambersons was at the bottom of the list, having just squeaked in at just over a million. In his book on Kane, Carringer states:
"...The Magnificent Ambersons had gone drastically over budget- $1,013,760.46 in actual costs against an estimate of $853,950."
Now, assuming this means production costs only, there would still be the costs of prints, distribution, and promotion. Nevertheless, I am impressed that the film made back almost all of it's production costs minus the difference between Carringer's figure and the just over a million figure of Variety's. And much of the promotion and distribution for the film would have been done by people already on the RKO payroll, and in RKO departments.
If the Variety figure is accurate, it seems Ambersons, while not a profit maker, was also not the box office disaster that everyone remembers it to be, and had enough of a reputation to get four Oscar nominations, including one for best picture.