Complex sentence: Gorbachev had set aside 5 billion rubles (about 8 billion U.S. dollars) in funds for a start on what would likely be a recovery cost that would exceed the cleanup bill for the 1986 Chernobyl disaster.
Split into simpler sentences: Gorbachev had set aside 5 billion rubles (about 8 billion U.S. dollars) in recovery funds. At that time it was a beginning on what would likely be a cost that would exceed the cleanup cost of the 1986 Chernobyl disaster.