Can't give you exact data (hell, you trying finding data for these types of things from clearly before the era of the internet alleviated these things! :lol), but here's what I could find for you.
http://www.gamedaily.com/articles/features/project-51-to-explore-castlevania-storyline/69268/?biz=1
This link that Wiki likes to cite for its "multimillion selling game series" article goes on to state that Konami claims that the series has sold 20 million units to that date (2006), from the series introduction in 1986.
Now, where do you suppose all those "millions" came from, assuming the claim is true? Likely not from the games were IGA was the producer at that point in time. If we go by the combined total sales of his GBA/DS stuff alone, with no re-releases, we'd be lucky to even get a half-million from all that. And that number would have to account games like the Saturn version of SOTN and Rondo of Blood, the latter of which, for the longest, was a JP-exclusive, on a console that furthermore, was really only relevant in Japan.
Is it that hard to believe that the series USED to be one with some serious clout to its name? I know, it's been a long time, and CV has since been a very "niche-y" franchise that's catered to a small, dwindling yet devoted fanbase for the better portion of a decade. But there WAS a period in history when CV used to be a major player in the Konami arena. And if that "20 million" figure is anything to go by, it represents a time when Castlevania used to have worldwide appeal that it basically doesn't possess any more.
In Japan, the PS3 version of Bayo debuted at the top of the charts its first week, and even the 360 version debuted at number 7 (no small feat, considering the 360's lot in Japan)....that's not "too hot"...? Especially for a new IP, at that?