What was not among whom? What did O'Henry want us to know from this line?
The author says this line after Jim hugs Della out of affection after seeing the magnitude of her sacrifice (her hair, which she parts with) for him.
The paragraph is:
Out of his trance Jim seemed quickly to wake. He enfolded his Della. For ten seconds let us regard with discreet scrutiny some inconsequential object in the other direction. Eight dollars a week or a million a year - what is the difference? A mathematician or a wit would give you the wrong answer. The magi brought valuable gifts, but that was not among them. This dark assertion will be illuminated later on.
I can't interpret this one.