Hiding in a snowdrift
Your math is off. Distributors take half the box, so that 1 billion becomes 500 million. Split between the studios is then 250/250, for a profit of 100 million each.Disney's current deal is 5% of the gross. I suspect this is because Disney didn't want to run into Hollywood accounting where, gosh, that billion dollar movie just wasn't able to turn a profit. It's unclear how much of the production costs Disney paid to make the movies, but it was clearly less than 50%, since their offer was to split production costs 50/50 and then split the take 50/50. That would almost certainly have to be gross again.
If the movie makes $1 billion gross, but it cost $300 million to make and advertise, split between Disney and Sony evenly, that would leave a net of $700 million.
If they split the gross, each studio gets $500 million and they spent $150 million, so they each profit $350 million.
If they split the net, each studio would get $350 million, and they spent $150 million, so they each profit $200 million and also there's $300 million unaccounted for.
Even splitting the gross is a huge difference from the previous deal, where even if Sony paid the full amount of production and marketing, the billion dollar movie with a $300 million budget would give Sony $650 million profit and Disney $50 million.
Interestingly, the numbers I pulled out of my ass here, if Disney put up 50% of the production and marketing cost, but got 20%, the profit for each studio would work out to $650 million for Sony and $50 million for Disney--the same as if Disney put up no money but got 5%. Which is a meaningless coincidence, but ultimately I think what will happen is both studios will go back to the table and they'll try to find a sweet spot where Disney assumes more of the risk, gets more of the reward, and maybe gets access to some more characters, because they have the actual numbers and they can tinker with them until they get something they can both live with.
Heck, if Disney ends up basically the same financially, but they can use more Spider-Man characters in MCU movies, they might consider that a win.
edit: also, for the record, Spider-Verse made 376 million on 90 mil (budgeted, w/M&A probably 135), so 376/2=188 less 135 for a profit of 53 million.