The real question: how much does a channel earn after the first sale?

A business owner should not ask only, “How much does a lead cost?” The better question is, “How much value does this channel leave once acquisition, retention, and reactivation are included?”. A cheap click can still be a bad deal if customers do not come back, or if the channel mostly attracts one-off buyers.

So the useful angle is lifetime net value, not contact cost. This fits a point often made by the OECD: digital tools can reduce search, distribution, and service costs, but they do not automatically reduce the costs of trust, conversion, and relationship maintenance.

  • Compare channels on cumulative margin, not raw volume.
  • Separate first purchase, repeat purchase, and reactivation.
  • Do not mix channels unless you can attribute sales consistently.

A simple four-column method

Take each channel over the same period, for example 90 days, and measure four blocks: total channel cost, customers acquired, gross margin from the first purchase, and then the share of customers who buy again within a defined window. CNIL notes that the consent or opt-out logic for commercial prospecting depends on the channel, so you should also include the operational compliance cost, especially for email, SMS, and partner-based prospecting.

A practical formula is simple: channel net value = gross margin from attributable sales + repeat-purchase margin + reactivation margin - marketing cost - sales cost - service cost - compliance cost. If you want a second lens, calculate payback time: how many months until the channel covers its own cost? A slow-paying channel can still be excellent, but only if your cash flow can support it.

  • Total cost: media, tools, sales time, creative work, and discounts.
  • First-sale margin: revenue minus direct cost of delivery.
  • Retention: repeat rate, frequency, average order value, churn.
  • Reactivation: campaigns, offers, sales follow-up, support.

Fictional example: four channels, one decision

Fictional example for a B2B SME selling a recurring service. Over three months, it compares four channels. Channel A: SEO, 18 customers, total cost €4,500, first-sale gross margin €6,300, and 6 customers bought again, generating €2,100 in extra margin. Channel B: trade show, 12 customers, total cost €9,000, first-sale gross margin €5,400, and 5 customers bought again, generating €2,500. Channel C: email to an existing compliant customer base, 20 customers, total cost €1,800, first-sale gross margin €4,000, and 10 customers bought again, generating €4,000. Channel D: purchased leads, 25 customers, total cost €7,500, first-sale gross margin €6,000, and only 2 customers bought again, generating €800.

The lesson: Channel D produces the most immediate customers, but its net value is weak. Channel C looks less dramatic, yet it wins on low cost, repeat purchase, and legal control. CNIL also points out that customer relationships tend to work better when unsubscribes and preferences are handled properly.

  • The best channel is not always the most visible.
  • A weak repeat rate can destroy an apparently good CAC.
  • A well-managed customer base is often more valuable than unqualified new traffic.

Limits you should respect before comparing too quickly

First limit: attribution. If a customer sees an ad, receives an email, and then calls sales, the winning channel depends on your counting rule. Keep one simple rule and use it consistently; comparability matters more than statistical perfection. Second limit: sales-cycle maturity. A trade-show channel may sell slowly but retain better, while a digital channel may convert quickly but fade fast. Third limit: legal framing. According to CNIL, some prospecting uses require consent, while others rely on opt-out or specific conditions, especially in B2C and when data is shared with partners.

In other words, a “profitable” channel that forces compliance shortcuts or inflates customer-service problems is a false bargain. The real test is not “does it work?” but “does it stay profitable once customers return, complain less, and cost less to serve?”.

  • Set an attribution rule before you compare.
  • Measure long enough to capture at least one repeat purchase.
  • Always add service and compliance costs.

Seven-day checklist: decide without comforting yourself with vanity metrics

Day 1 — Owner: sales director. Observable result: list of channels to compare and one common period selected. Day 2 — Owner: marketing. Result: complete costs gathered by channel. Day 3 — Owner: finance. Result: gross margin per channel calculated on attributed sales. Day 4 — Owner: customer service. Result: repeat-purchase rate, return reasons, and support volume compiled. Day 5 — Owner: DPO or compliance lead. Result: CNIL rules by channel and key risks documented. Day 6 — Owner: general manager. Result: comparison table with net value, payback time, and uncertainty. Day 7 — Owner: sales director. Result: documented decision to scale two channels and reduce or fix one.

If you keep only one metric, keep net value per acquired-and-returning customer. It is less glamorous than click-through rate, but far more useful when payroll is due.

  • Final deliverable: signed comparison table.
  • Decision: scale, correct, or stop.
  • Success indicator: the same comparison can be repeated next month.