If you use exchange feature you don't have to pay interest. If you borrow money to buy the 100 BTC then you need to pay 25 cents per day at current interest rate.
yet right now a short pays nothing. i am not clear why borrowing btc to sell also does not cost something let alone pays most of the time.
how do you calculate such things?
edited to add: i just sent some btc from bitcoinica to an address and it has said pending for the last 42 minutes... odd that something has
to sit like that for so long. I should have seen it at least hit the network right away or are you doing something manually?
BTC costs less to get (depending on available customer deposits) and USD costs more. The interest rate difference determines the swap.