<?php
$string = '...nomaterwhat13124123,"321,00",,nomaterwhat
...nomaterwhat12321,"322,20","134,00",nomaterwhat
...nomaterwhat1321,"211,00",,nomaterwhat
...nomaterwhat31313,"241,00",,nomaterwhat';
echo preg_replace("/,(\"\d+,\d{2}\"),,nomaterwhat$/m", ",$1,$1,nomaterwhat", $string);
PD9waHAKJHN0cmluZyA9ICcuLi5ub21hdGVyd2hhdDEzMTI0MTIzLCIzMjEsMDAiLCxub21hdGVyd2hhdAouLi5ub21hdGVyd2hhdDEyMzIxLCIzMjIsMjAiLCIxMzQsMDAiLG5vbWF0ZXJ3aGF0Ci4uLm5vbWF0ZXJ3aGF0MTMyMSwiMjExLDAwIiwsbm9tYXRlcndoYXQKLi4ubm9tYXRlcndoYXQzMTMxMywiMjQxLDAwIiwsbm9tYXRlcndoYXQnOwplY2hvIHByZWdfcmVwbGFjZSgiLywoXCJcZCssXGR7Mn1cIiksLG5vbWF0ZXJ3aGF0JC9tIiwgIiwkMSwkMSxub21hdGVyd2hhdCIsICRzdHJpbmcpOw==
...nomaterwhat13124123,"321,00","321,00",nomaterwhat
...nomaterwhat12321,"322,20","134,00",nomaterwhat
...nomaterwhat1321,"211,00","211,00",nomaterwhat
...nomaterwhat31313,"241,00","241,00",nomaterwhat