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The trade only pays if the subscription changes behavior. A customer who would have reordered anyway now pays less for the same orders, and the discount is pure cost. A customer who would have lapsed and now keeps ordering is where the value is. The question is how much of each you have, which a cohort comparison against one-time buyers can answer.

A worked example, as arithmetic. A $40 product carries 60% contribution margin, $24 an order. With a 15% subscription discount the order is $34 and contributes $18. If one-time buyers place an average of 1.8 orders a year ($43.20 of contribution) and subscribers place 5 ($90), each subscriber is worth about $47 more a year, which is the extra CAC a subscriber can carry. If subscribers only place 2.2 orders, they are worth less than one-time buyers ($39.60), and the program costs money.

Subscriptions also change what acquisition should optimize for. A brand selling mostly on subscription should judge channels by subscriber retention at month three or six, not by first-order ROAS, because two channels with identical first-order costs can produce subscribers with very different lifetimes.