About the PC version, in the leaderboards there were 3091 entries for Green Hill Act 1 earlier today, and according to the steams achievements page, 20.4% of players participated in the leaderboards.
This means that the PC version so far sold over 15150 copies (I say "over" because most likely there are players who have the "joined the ranks" achievements who might not have an entry in Green Hill Act 1, they could have participated in a random stage just to get the achievement).
Anyway, most likely both the PS3 and XBox 360 versions sold like 10x more or so, but still, over 15000 copies in 2 weeks for a game of a franchise that is not popular on the PC that was released in a month with many other popular releases, I guess it's not too bad.