The purpose of listing this information was primarily for public declaration, as opposed to question how they may have acquired the money to purchase houses in California. This is Sri Lankan law that was circumvented by the Rajapaksa clan by simply introducing laws tailor-made to their needs.
Although eyebrows may be raised as to how such properties may have been purchased considering their combined lack of professional qualifications or educations and/or experience in business. Also, please keep in mind the $4.5M figure used is an AVERAGE for a house of similar size & bed-rooms, in very close proximity. Exact price of this house can vary up or down, although not significantly.
While mortgages are an obvious and most probable option, as a rule of thumb credit for such ventures are only given such that payment of debt is no more than 30% of disposable household income. Payment of debt includes payment for credit cards, automobile, child-support, student loans, etc, etc.
A simple reverse calculation for a $4.5 million dollar mortgage, on a 30 year loan at an interest rate of 4% will require a monthly payment of $21,000. If this represents 30% of disposable household income, then full monthly income is $70,000. This converts to $840,000 annual income.
Since the United States does not have a tax treaty with Sri Lanka, it would be interesting to know whether taxes for this income has been paid.