Noting the subscription wording on some systems: Subscriptions cost $xx at the end of each month, but only if the system has made profitable trades.
This sounds like even if just one trade is profitable (like out of 20, etc.), then you will be billed. Think you mean overall profitable during that billing period (week, month, quarter). Might be better to phrase this as "…but only if the system is profitable for the (billing period)"