What is the effective margin rate of interest for taking on a
leveraged position through stock index futures?

The margin rate when buying stock on margin is the interest rate that
has to be paid for borrowing the money to buy the stock.  But it is
possible to construct a replicating position by maintaining a long
position in a stock index future.  For example instead of buying the
Russell 2000 ETF on margin, you might take a long position in a
Russell 2000 e-mini future contract.  To take on this future position
requires placing with
the broker a performance bond.  But in addition to the lost interest
on the performance bond, investors implicitly pay interest in the
premium of the future price over the spot price.  Has anyone estimated
this effective cost in terms of an percentage rate of interest?  The
answer may involve figuring out what arbitrageurs demand as the
implicit interest rate
that drives the fair value premium to zero.