The cost of margin, like most loans, is imposed in the form of interest costs. The interest rate you'll pay depends on the size of the loan and the brokerage. As the debit size increases, the interest rate decreases. For example, TD Ameritrade charges an effective interest rate of 9.5% on debit balances under $10,000 and 7.5% on debit balances over $250,000. These rates will fluctuate with the broader interest rate environment.