To be exact, "The Cage" cost $615,781.56.00.
"Where No Man Has Gone Before" cost $354,974.00.
The decrease in cost can probably be credited to the second pilot being shorter (50 min. vs. 78 min.), start-up costs not incurred by the second pilot (sets, costumes, props, research/development, etc.), and the crew having the experience of the first pilot under their belt.
Except that The Cage
is 63 1/2 minutes, not 78, and the original "pilot" cut of WNM is 52 1/2 minutes, so it's only 11 minutes shorter.