The amount financed is the net amount of money you are borrowing from the lender, minus most of the upfront fees the lender is charging you. (consumerfinance.gov)
When money is borrowed, interest is typically paid to the lender as a percentage of the principal, the amount owed to the lender. (money.stackexchange.com)
In your example, it would depend on how much money you're borrowing from your broker on margin. (money.stackexchange.com)