ICv2 reported that the sales of manga in US for 2008 was 175 million dollars and showed a drop by 17% compared to 2007. The total sales of the graphic novels including American comics increased by 5% (2007: $375 million; 2008: $395 million).
Here is the chart of the sales of comics in US from 2004 to 2008.
The number of manga titles released
2005 1088
2006 1208
2007 1513
2008 1372
2009 1224 (estimation)
Source: ICv2
Source: Anime!Anime!
Related Topic: Has Manga Market reached a plateau?