A Panamax container ship, not the largest, can burn 10 metric tonnes an hour of bunker oil, more or less.[2]
Very Low Sulfer Fuel Oil (VLSFO) is running, depending on where the ship is loading up, 850 to 1150 USD a tonne.(July 7 2022)[1]
In round numbers, say, USD 1,000 a tonne, USD 10,000 an hour, USD 250,000 a day, and for a 20 day trip, USD 5,000,000.
[1] https://shipandbunker.com/prices
[2] https://www.freightwaves.com/news/how-many-gallons-of-fuel-d...