Actually I figure this way.
About 30% of Americans are pasionately Christian, the core audience for "The Passion of the Christ" and even "The Chronicles of Narnia: The Lion the Witch and the Wardrobe." About 3% of Americans are gay, the core audience for "Brokeback Mountain." Both groups probably have allies numbering about 2.5:1 (meaning 75% of Americans approve of Christianity as an organized religion, and maybe about 7-10% of Americans are "gay-friendly".)
By such reckoning, the ratio of sales for The Passion and Narnia to Brokeback Mountain should be about 10:1. The Passion made $373 million; Narnia could come near $300 million. That would leave BM at about $30 million. I don't think its going to make that, let alone cross over into the general public.
(Oh, yes, I know that over 80% of Americans identify themselves as Christians, but many of them vaguely believe in a God, are merely nominal, etc...)
Actually, your calculation is just about right. However, I believe Brokeback Mountain will do better than your $30M prediction becasue all left-wing Democrats will go see it to advance the "gay" gender and aceeptance. However, like Michael Moore's film which garnered $125M, it did absolutely nothing to help John Kerry gain the POTUS. Fact is, Mr. Moore and John Kerry were soundly defeated by a meager $100K ad place on limited TV by an organization known as the Swift Boat Vets.
That would leave BM at about $30 million.
If it does significantly more than $30 million, are you going to have to revise the numbers of gay, and gay-friendly Americans?
Both groups probably have allies numbering about 2.5:1 (meaning 75% of Americans approve of Christianity as an organized religion, and maybe about 7-10% of Americans are "gay-friendly".)
A little problem with your assumptions here, even if I do give you your numbers (which I strongly doubt). The number of people who are tolerant of hard-line Christianity has no bearing on how many people went to see "Passion". The word on "Passion" was out, too, and if you really wanted to help Mel Gibson make a statement, you bought a ticket to that movie, if you really didn't want to see a man being tortured to death, you stayed away.
Word is out on BBM, too, the gay sex scenes are not considered graphic, there is a considerable amount of female nudity instead, and there are positive reviews of the acting, the story, and the cinematography. Going to see it will not mark a man as gay, at least in most suburban American cities.
Box office is already starting to grow on this picture, frankly, as it seems to be a political statement from the left in answer to the "Passion" numbers, I fully expect it to do as well as Narnia, and possibly as well as "Passion". Anyone who is cheered by last week's numbers is going to be painfully disappointed within a month.