I'm not a ksh user, but this script seems to work for me.

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cat ~/.local/bin/commafy

#!/usr/bin/perl -nl
$_ =~ s/(\d)(?=(\d{3})+(\D|$))/$1\,/g;
print;
[download]

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My usage (to find out that a recent nix build used 18 GB of space):

$ find /nix/store -cmin -150 | perl -nle '$sum+=-s "$_";END{print $sum
+}' | commafy
18,025,136,505
[download]

2018-12-16 Athanasius added code and paragraph tags