For any who come across this series, allow me to forewarn you: it was never completed, so the last available chapter leaves you hanging in the middle of the story.
Which is a pity, because the story has stuck with me, so there was potential for it to be something.
As with all my manga reviews, this covers all 10 volumes of the series and will contain spoilers but I'm guessing you want to know where this series cuts off so you don't have to bother investigating the 10 volumes that were published, so proceed to sate your curiosity.
The premise follows Kanako, a sweet, glasses and braid wearing 16 year old girl who is picked on at school and mistreated by her family at home, and her experiences after she sacrifices herself to marry a wealthy man, Taiga, to settle her family's one billion yen debt. Throughout the series, Taiga picks on Kanoko while also treating her better than her family, so she falls in love with him, only to be swept up in the drama of his family and past: his cousin, Reo, who looks identical to him, blames him for the death of his sister, and takes Kanako to be a substitute for his mentally ill mother, who we learn sexually abused Taiga as a child, and Reo eventually falls for Kanako, fighting Taiga to keep her.
The series ends on "part 1" with Kanako going home with Taiga despite Reo revealing that Taiga was the one who put her family in debt and used it to manipulate a marriage out of her family, consummating their marriage because she loves him anyway, and then running away, asking that he not look for her--perhaps implying that she's going back to Reo because of a previous promise she made him out of pity for all his suffering, but it's uncertain what would have been tackled in part 2.
The art has really nice 90s kinda aesthetic (it was published in the early 2000s) and was entertaining with its darker themes, plus Kanako is a really cute heroine, so the potential was all there.
Of course, the fact that Kanako is a 16 year old forced into marriage with an adult man with lots of money and influence is, yet again, a rotten thing that's too common in manga, but overlooking that, I enjoyed the story and think it had potential.
I couldn't find out why the story was never finished, but, looking into the other series done by the artist, she follows a similar theme in several other works (Trouble Wedding, M-Shiki Princess) it could be that she simply didn't care for the direction Billion Girl had taken and didn't want to pursue.
According to the Japanese wikipedia article, the "final chapter" was released in a later edition published in 2014, but I couldn't find anything about it beyond that, so perhaps there is a technical ending out there and it just didn't make it into English (considering fan groups stopped working on it around 11 years ago).
Needless to say, I imagine if the author made just 1 more chapter to round things off, it's not hard to guess what was written in it nor be a bit miffed at all the loose threads that couldn't have been addressed in the pages of a single chapter.