SoftBank Group added $15 billion to its market capitalisation on Thursday after CEO Masayoshi Son unveiled a plan for the company's biggest-ever share buyback worth $5.5 billion. Further, Son's networth rose $2.6 billion, bringing his fortune to $16.6 billion. Son argued the company's 25 trillion yen assets minus 4 trillion yen debts didn't reflect its 9 trillion yen market value.