cat ~/.local/bin/commafy

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

##</code><code>##

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