Well if a single primary school needs 1 bln or anything close to it to
  operate that won't be scale able.

Just to clarify, Zuckerburg has spent over a billion on education related grants and other gifts since 2010, but no one knows what he has spent on this one school. I can't see how he could have spent anywhere close to a billion dollars. Unfortunately the writing is fairly poor so I'm not even sure how many students this school will serve from which to guesstimate a cost.

FTA:

  Chan did not tell the Mercury News how much she and her husband are contributing
  to The Primary School, but when it is fully built, it will serve 50 students 
  in 14 grades (pre-K through 12) plus the families of their 700 students.