itunes ad on you tube

I was just wondering when you watch a video on utube and a "buy this song on i-tunes " comes up at the bottom of the screen do both utube and itunes take a cut of that sale.Does this in turn result in the artist receiving less income from that sale conpared to someone buying the track direct from itunes?
if this is the case does anyone know the %’s involved
thanks
haddow20

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