Price check, pre-WWII OU's vs sxs. OU's first (1940):
Rem 32: $127
Browning Superposed Grade 1: $90
(Both the above with SST and ejectors.)
Marlin 90: $40
Savage 420: $35
(Both the above with DT and extractors.)

SxS:
Win 21: $106
Parker VHE: $170
LC Smith Field Grade: $74
Fox Sterlingworth: $87
(All above with ST and ejectors)
Fox Model B: $26
Hunter Fulton: $30
Hunter Special: $35
Win 24: $30
Iver Johnson Hercules: $35
(All above with DT and extractors)

So pre-WWII OU's wern't necessarily more expensive than sxs, if you compare guns with similar features. But Jim's got it right: Back before WWII, production volume favored the sxs; it now favors the OU's, by a HUGE margin. That's the main reason they're cheaper.