The commission is 0.025% of the value used, charged when you open a trade. Value used is your margin multiplied by your leverage. You pay it once, when you open the position. Closing it costs nothing.
Margin | Leverage | Value used | Commission to open |
$1,000 | 1:10 | $10,000 | $2.50 |
$1,000 | 1:50 | $50,000 | $12.50 |
$2,000 | 1:50 | $100,000 | $25.00 |
Because the commission is based on value used, using more leverage on the same margin means a higher commission.