The commission rate is consistent across ALL currency pairs, including XAU. This is what it means to have the commission expressed as USD per USD million traded (which can also be thought of as EUR per EUR million traded, etc.). This commission is charged in your account currency.
Commission example: Assume spot rate of 1.10 for EURUSD. A client trades 100,000 EURJPY in his USD account on which he has selected to pay 30 per million. EUR million traded = EUR 100,000 / 1, 000,000 = 0.1. EUR commission = EUR 0.1 * 30 = EUR 3.00. USD commission = EUR 3.00 * 1.10 = USD 3.30. The standard conversion fee is applied when commissions are converted into the account currency.