Sega put out some sales numbers for the series, 2.5 million so far since the 1st project diva game. Would like to see some NA sales figures on diva f, it probably did okay i would assume since they are localizing diva f 2nd as well.

• Hatsune Miku: Project Diva (PSP) – July 2, 2009 – 187,558
• Hatsune Miku: Project Diva 2nd (PSP) – July 29, 2010 – 367,364
• Hatsune Miku: Project Diva Extend (PSP) – November 10, 2011 – 283,040
• Hatsune Miku and Future Stars: Project Mirai (3DS) – March 8, 2012 – 180,696
• Hatsune Miku: Project Diva f (PS Vita) – August 30, 2012 – 217,028
• Hatsune Miku: Project Diva F (PS3) – March 7, 2013 – 160,219
• Hatsune Miku: Project Mirai 2 (3DS) – November 28, 2013 – 134,785
• Hatsune Miku: Project Diva F 2nd (PS3, PS Vita) – March 27, 2014 – 82,150 (PS3) / 131,877 (PS Vita)

Link

Considering that every game in the series after the first one has been made extremely cheaply that's not bad.