For Avengers: Infinity War, it’s plausible, especially if this is indeed an overlap with Avengers 4.
Spending $1 billion on two movies increases the chance of financial success between each of them, and that’s not even accounting for merchandise sales.
That said, let’s hope that this $1 billion price tag applies to both Infinity War and Avengers 4, because only four of Marvel’s movies have cracked the $1 billion mark worldwide, and the studio will want to make as large a profit as possible.
For most movies, including major blockbusters, seeing a $1 billion price tag would be absurd.