There is no daily cost. You leave collateral in BTS tokens, take your bitUSD, convert them to real USD and go spend, take a vacation, play casino, hang up with chicks. Pay back your debt at your convenience. No interest. But, if the value of your BTS collateral falls due to volatility, your collateral will be sold at very bad price for you to force you to pay off your debt. Think of it as of overall cost of your loan.