As we know, the Hollywood scenes with Hagen and Woltz were shot with doubles for budget reasons.
But was a flight from New York to California for two actors in 1972 really that expensive?
Does anybody know the prices?

It would have been more than two actors. Either FFC and some of the camera people would have to go to Holllywood to film a locatiion scene at some mansion in Bel Air or Beverly Hills, which would have cost money, or FFC could have left it to a second unit director and crew. I doubt he would give up that much creative control over those scenes, so between that and the tight budget it makes sense they filmed it in New York. Remember the studio thought this movie was going to bomb and they were always on the brink of firing FFC and Al Pacino so rocking the boat to shoot that scene in LA was not in the cards.

I agree with dt. They used stock footage of Hagen's airplane landing, Grauman's Chinese Theater, and parts of Paramount's lot. But the Woltz sequence was short and limited in sweep, so there was no need to incur the expense of shooting in Hollywood.

Contrast with the Cuban sequences in II: by that time FFC had carte blanche and the Havana interlude was a lengthy, major part of the movie. FFC had a real need for maximum authenticity, which was why they did it in the Dominican Republic, and damn the cost. Serious professors of Cuban history urge their students to view GFII for an accurae depiction of what Havana was like on the eve of Castro's takeover.

FFC's obsession with authenticity was one of his major strengths as a director. In the documentary, "Hearts of Darkness," about his filming of "Apocalypse Now," we see him directing the French plantation scene (which was deleted in the theatrical release but restored in "Apocalypse Now Redux"). FFC is setting up a dinner table for the French planters and the patrol boat crew. FFC says, "I want a bottle of Puligny-Montrachet [top-line white Burgundy] and I want it served at 48 degrees. I want any Frenchman who was in Vietnam at that time and who sees this movie to believe it." I thought, who the hell watching this movie would know if it wasn't Puligny-Montrachet served at 48 degrees? Then I figured it out: a director who sweats such tiny details is going to get everything right.

Great obswervation TB, but how would French Colonialists living up the Mekong, in the jungle, during a war be able to maintain the Montrachet at cave temperatures?

I was referring to the scene where Woltz shows Hagen his palace. Wasn't this filmed by FFC? "The Godfather Annotated" mentions a "second unit" at the estate of Harold Lloyd. Does that mean, FFC wasn't there?