📩 The metric that separates winners from wasted spend

People love to brag about how cheaply they buy leads. It sounds impressive in a screenshot. But CPL on its own is almost useless.

What really matters is activation rate, meaning how many subscribers actually engage with you by opening, clicking, or replying. And even that is only half the story.

The bigger picture is simple: if you scale list growth without protecting deliverability, the whole machine falls apart. Take your eye off that balance and things start to slide, activation drops, inbox placement slips, and revenue goes with it.

If you want to know which traffic sources are actually profitable, you have to go one level deeper and measure how long subscribers from each source stay active on your list.

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A quick note before we dive in

Before we get into the numbers, one quick thing. There are acquisition partners out there that focus on warm, signal-verified subscribers rather than cold, spray-and-pray traffic. Those sources tend to cost more per lead on the front end but are already primed to engage with content like yours, and they can scale without wrecking quality.

Keep that idea in mind as we walk through the math, because it is the only way a higher CPL can still be the better deal.

Alright, let us break it down. 👇

Step 1: Look at the real cost of engagement

CPL is only interesting once you tie it directly to engagement.

Take a simple example:

• Source A: 1.50 dollar CPL, but only 10 percent ever click. That works out to 15.00 dollars per engaged subscriber.

• Source B: 2.50 dollar CPL, but 40 percent become clickers. That is 6.25 dollars per engaged subscriber.

Once you calculate Cost Per Engaged Subscriber, the “expensive” lead starts to look like the bargain.

Of course, sometimes a cheaper lead can still win. If a low cost source also holds its own on engagement, you should know that too.

The point is not to be clever with acronyms. You simply need to know what it really costs you to get one genuine click from each source.

👉 CPL is just a vanity stat unless you know what share of those leads actually engage.

Step 2: Measure how long subscribers stay active

This is where profitability really lives.

It is also where most media and newsletter operators struggle, because very few ESPs hand you this view in a clean report.

One big exception is an operator like Matt Paulson over at MarketBeat, who has built deep reporting around this. Most teams are not there yet.

The real return on your acquisition spend comes from understanding how long a lead from each source stays active on your list. That is your Average Length of Engagement, or ALE.

Imagine this:

• Source A and Source B have the same Cost Per Engaged Subscriber.

• Subscribers from Source B keep opening and clicking for roughly twice as long.

Which source wins? Source B, every time.

To calculate ALE, you need to treat each source as a cohort. Look at subscribers acquired from that source during a defined time frame, then track how many of them are still engaged later.

A simple way to think about it:

1. Pick a month and a source. Count how many engaged subscribers you have from that source in month 1.

2. Come back later and check how many of those same subscribers are still engaged.

3. Use those counts over time to estimate how long, on average, a subscriber from that source stays active.

Important: when you pull the numbers at a later date, you want to know how many subscribers created in month 1 are still actively engaged today, not just how many new people you added since then.

The more time passes, the better your ALE estimate becomes. Twelve months of data beats six.

If you want an easy way to run this, you can build a simple spreadsheet with:

• Source name

• Cohort month

• Active engaged subscribers by month

• Calculated ALE for that source

Reply to your own list with something like “Show me the ALE” and send them that sheet, if you want your readers to play along.

Step 3: Add the LTV layer

Two sources can show the same cost per engaged subscriber and still be very different in value.

If one group stays active twice as long, that source is far more profitable, even if the front end numbers look identical.

That is why you need to look at subscriber lifetime value, source by source.

Here is the basic math:

• CPL (Cost Per Lead) = total cost ÷ total leads

• CPES (Cost Per Engaged Subscriber) = total cost ÷ engaged subscribers
  (engaged subscribers here means people who have clicked in the last 30 days)

• RPES (Revenue Per Engaged Subscriber, monthly) = monthly revenue ÷ engaged subscribers

• Subscriber LTV by source = RPES × ALE

• Profitability ratio (LTV ÷ CPES) = LTV ÷ Cost Per Engaged Subscriber

Once you have these, you are no longer chasing cheap leads for screenshots. You are looking at true subscriber value, source by source, and putting your budget where the math works.

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Step 4: Watch activation by ISP

Not all leads behave the same across inbox providers.

Gmail might love a particular source. Outlook might be suspicious of it. Yahoo could drive strong engagement while Comcast barely stirs.

When you map activation rates by ISP, you start to see which sources quietly raise your reputation and which ones drag it down.

That view lets you scale the right sources and dial back the ones that poison your sender reputation, even if their surface-level stats look decent.

Step 5: Understand the risk of scaling too fast

Even if you find a golden source, you cannot just slam the gas.

Inbox providers do not like sudden, sharp spikes in volume. When you push too hard, too fast, a few things happen:

• Deliverability slides.

• New leads turn into dead weight because they never see you in the inbox.

• Even previously engaged subscribers stop seeing your messages.

• Revenue, which looked great in the short term, falls off a cliff.

On paper, it can look amazing for a moment. Low CPL, rapid list growth, big numbers on a dashboard.

A couple of weeks later, inboxing collapses, engagement craters, and the revenue that used to feel reliable suddenly disappears.

You did not just waste money acquiring new leads. You also damaged your ability to monetize the list you already had.

Sustainable list growth is not about cheap names or vanity numbers. It is about:

• Tracking activation rate by source, not only CPL

• Scaling gradually enough to keep inbox placement healthy

• Mapping engagement at the ISP level

• Knowing how long each source stays active

• Calculating LTV by source so you can see real profitability

• Keeping deliverability front and center as you grow

The marketers who win are not the ones with the biggest lists. They are the ones with the most profitable, most engaged lists, because they understand the real math behind subscriber value.

Final thought

Inbox placement is the great equalizer.

You can pour as much budget as you like into acquisition, but if subscribers do not activate and then stay active, you are just renting names on a spreadsheet, not building a durable business.

The brands that win know their numbers cold. CPL, activation rate, ALE, LTV, and deliverability health. When those are in line, growth stops feeling like a gamble and starts looking like a system you can actually trust.

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