800k in the US. I almost forgot about the US recession. This will actually depend on how the fight will be marketed. A good marketing can do the trick.
The more I think about it ... I don't think that this fight will exceed 800k buys ... which isn't bad ... but I don't think that the american public is going to be drawn to this figh like that.
With the economy in the states, I am going to say close to 400-600k, I don't see the casual fans dishing out the money to watch this fight.
I don't see this fight being that big of a draw ... there aren't too many PPV studs around right now ... I can see this fight tanking like Pavlik v Hopkins or Jones v Calzaghe