To quote Jebb, from his note on this line:
Literally, "Not to be born exceeds every possible estimate," of the gain, as compared with the loss, of being born ... Rate the gain of being born as high as you please; the gain of not being born is higher. Two other ways are possible: —(1) "Not to be born excels the whole account, —i.e., excels all the other things (joys, sorrows, of life) that come into account. The drawback to this is the somewhat strained sense of λόγον. (2) "Stands first on the whole reckoning," (τὸν ἅπαντα λόγον being cognate accusative, or accusative of respect) —i.e. when a balance is struck between the good and the evil of being born. This seems too cold and cautious for the context.