The difference in price between 3496 and 3204 is 292 points.
The market moves as you predicted, up to a level of 3500, at which point you decide to close your position and take a profit. The sell (or bid) price after the spread is applied is 3496. The difference in price between 3496 and 3204 is 292 points. This, excluding other costs, brings your profit on the trade to $2920 – a return of 19.4% on your margin deposit.