1. The Grand Mosque, Mecca — $30 billion+ (£20.5 billion+). The world's most sacred mosque is also the world's most expensive construction, though with so many upgrades it's impossible to put an accurate figure on it. In 2011, a $10 billion project to add over 4 million square feet of space was announced. It currently covers over 88 acres and hosts over 5 million Muslims every year.