The price varies widely depending on the composer (sometimes even type of track); you'd have to find the ones you're interested in and ask. It's often rated as $/minute of music.
As for the CC license, as far as I know there're different types of it; e.g. there's the Creative Commons Noncommercial which obviously doesn't allow it (though you could still contact the artists themselves to request the permission).
I think what you might also be looking for is royalty-free music:
http://en.wikipedia.org/wiki/Royalty-free_music [
Edit: Oh, I re-read and it's more an issue of what category is sponsored flash games? My bad. Though I'd imagine since it's an established money-earning way of publishing the games, it'd count as commercial use.]