I'm just wondering why he would have to be cutting deals then if it's in FOx's studio?
It's probably about the government rebates. It's quite likely that Canada's movie production incentives and tax rebates would be much more appealing than those currently on offer from Australia. So you could have the situation where it could be more economical to film in Canada using a third-party's studio rather than Fox using their own studios in Sydney.
Scott might prefer the facilities or extra soundstage space that the Sydney studios offer and is trying to convince Fox to allow him to use it by sucking-up to the politicians and trying to get better incentives.
This is all purely conjecture though.
Rebates perhaps, but they don't typically worry too much about that with films of this caliber.
It is more likely due to finding an ideal location for both Interior and Exterior aspects of the shoot, that saves them production costs all around. For Interiors - set building in soundstages, its about the needs of the set and location to exteriors that correspond ie; finding a location for the soundstages that can accommodate their needs but is ideally located for the External -on location shooting as well while also saving them enormous salaries for actors. They will consider these things based on need and which actors schedules will factor into the shoot. For instance, if they have 2 months to use Michael Fassbender, and he will be needed for most of the interior and external shoots, it will save them millions of dollars to not have to do giant company moves to different countries or worse have multiple units in different countries. You can imagine that using Michael Fass for an extra 2 weeks might cost a lot...and then they will most likely have to arrange for reshoots and pickups anyway. They will most likely need to do this multiple locations to some extent anyway, but its a puzzle. Does the stage space accommodate the needs of the film, and can they get all of x actors scenes shot in y amount of time, without losing or wasting money, and will it be cheaper to fly crew in or use local? For something like this, department heads and above the line workers will make those decisions with the Line Producer and UPM who will be trying to please Ridley Scott because he will want certain things.