Paying for an option to use a story is much much cheaper than the full script payment.
This makes sense, and explains why the producers exercised so many "story cut-offs," but commissioned few teleplays that did were not produced.
Indeed, in the first season, I believe the only teleplays that were not produced were "From The First Day to the Last" (John D.F. Black's attempt at an "envelope" for the original pilot), "The Omega Glory" (Roddenberry's proposed script for the second pilot, and, of course, a produced episode in the second season), and "A Portrait in Black and White."
That makes the claim that the third season did not have the money to spend on scripts that would not be produced suspect, though, since the producers purchased at least three teleplays that year which were not produced (which I listed previously), in addition to exercising a number of story cut-offs.
EDIT: Although "He Walked Among Us" appears to be a teleplay carried over from season two, not the third season. Hmm.
EDIT 2: And "Shol" is a second season carry-over, too.
EDIT 3: And "The Joy Machine" is from the second season, too. Okay, that clears that up a little. It looks like they didn't shelve any teleplays that year -- just stories.
["Let That Be Your Last Battlefield" was developed from a junked first season script. By coincidence I happened to have the information for that episode up on my screen.]
That first season script was called "A Portrait in Black and White." It was written by Barry Trivers. I believe Coon's third season attempt at the same story (as Lee Cronin) had multiple titles; and for some reason I want to say that "Down from Heaven" was one of them. Not sure, though.