By John Revill and Silke Koltrowitz LUGANO, Switzerland (Reuters) -The cost of rebuilding Ukraine following Russia’s invasion could reach $750 billion and rich Russians should help pay the bill, Ukrainian Prime Minister Denys Shmygal said on Monday. “We believe that the key source of recovery should be the confiscated assets of Russia and Russian oligarchs,” he told a conference in the Swiss city of Lugano, citing estimates that frozen Russian assets were worth $300-$500 billion. “The Russian authorities unleashed this bloody war. They caused this massive destruction and they should be held ac…