Disaster Declaration Improvement Act of 2013 - Directs the Administrator of the Federal Emergency Management Agency (FEMA) to issue regulations to ensure that: (1) in any review of a request for major disaster assistance conducted by FEMA, the factor relating to the estimated cost of assistance (evaluating the estimated cost of federal and nonfederal public assistance against the statewide population) is provided consideration that is equal to that provided to each of the other factors (i.e., localized impacts, insurance coverage in force, hazard mitigation, recent multiple disasters, and other federal assistance programs); and (2) FEMA considers a state's request for major disaster assistance for recommendation to the President without regard to any per capita damage threshold (other than a threshold described below) in any case in which a disaster occurs with respect to a city or unincorporated jurisdiction with a population of less than 250,000, located in a county with a population of more than 1 million and a state with a population of more than 5 million, and with respect to which the financial damage relating to the disaster exceeds 10% of a city's general fund or $100 per capita (in the case of an unincorporated jurisdiction).