from anime news network...
"A presentation for Marvelous Entertainment's 2006-2007 financial report reveals that a sequel is in the works for its hit animated television series Gunslinger Girl about cybernetically enhanced girl assassins. The first 13 episodes were produced at a cost of 13 million yen (about US$110,000) each from 2003 to 2004. Yu Aida is still publishing the original manga in Media Works' Comic Dengeki Daioh magazine with eight compiled volumes and 46 installments so far."
(http://www.animenewsnetwork.com/news/2007-06-05/marvelous-entertainment-to-make-more-gunslinger-girl)