Officials told The Fact Checker that the president was referring to an internal Office of Management and Budget estimate of the potential liabilities over the life of the disaster that would need to be committed under the Robert T. Stafford Disaster Relief and Emergency Assistance Act of 1988. The estimate was described as a high-end estimate subject to change year by year.
Currently, the estimated Stafford liabilities amount to $50 billion. Adding the $41 billion in announced funding to the $50 billion in Stafford liabilities gets you to $91 billion.