I asked an architect, who was involved in the project, why the Sports Hub cost $1.3 billion?
He said that the Sports Hub is more than the stadium. There are 7 structures - national stadium, indoor stadium, arena, aquatic center, mall, office and a huge car park.
The actual cost of the national stadium is probably half of the total, i.e. $650 million (but this is just a guess). It is expensive because of the retractable roof that allows the national stadium to be used for all weather.
More details can be found here: