The Metric That Separates Winning Lists from Wasted Spend

👋 Hey friends,

Everyone loves to brag about how cheap they’re buying leads. But here’s the reality check: CPL by itself is useless.

The real metric that matters is activation rate—how many subscribers actually engage (open, click, reply). And even that’s only part of the story.

Because here’s the kicker: focusing on deliverability while scaling list growth is the only way to build a sustainable acquisition strategy. Take your eye off that balance and things unravel—activation drops, inboxing tanks, and monetization goes with it.

And if you want to know which sources are truly profitable, you have to go one step deeper: measure how long subscribers from each source actually stay active on your list.

Before we get into the details, a quick message from Audience Bridge…

…Let’s break it all down. 👇

CPL doesn’t mean much unless you tie it to engagement.

When you calculate Cost Per Engaged Subscriber, suddenly the “expensive” lead is the real bargain.

But maybe… a lower cost lead can still be the real bargain.

In the end, you just need to know what it’s costing you to get that click.

👉 CPL means nothing unless you know how many of those leads actually engage.

This is where profitability lives.

And it’s also the hardest thing for most media and newsletter operators to calculate.

Unless you're Matt Paulson (🙂Happy 40th!) at MarketBeat, that is.

The real ROI on lead acquisition comes from understanding how long a lead from a specific source stays active on your list (Average Length of Engagement).

Example:

Who wins? Source B.

The reason this is the hardest measurement to calculate, is that most ESPs don’t provide access to this information easily.

In order to measure the Average Length of Engagement (ALE), you need to track via cohort analysis how a source acquired during a specific time frame is still active.

If you track how many engaged subscribers you have from a source in month 1, you can then check how many are still engaged at a later date and use the Average Length of Engagement formula to project the ALE.

IMPORTANT: When pulling the numbers at a later date, you want to know how many subscribers created in Month 1 are still actively engaged today.

The longer the time has passed the more accurate your ALE becomes (i.e. you have 12 months of data vs 6 months).

🚨If you would like the Average Length of Engagement (ALE) spreadsheet calculator above, simply reply to his email “with Show me the ALE!”

And if you have any questions about it feel free to ask.

Two sources might look identical on paper with the same cost per engaged subscriber, but if one group stays active twice as long, that source is far more profitable.

That’s why looking at subscriber lifetime value (LTV) by source is critical.

Here’s the math:

This is how you stop chasing vanity numbers and start understanding true subscriber value.

Not all leads perform the same across inbox providers.

When you map activation rates by ISP, you start seeing which sources boost your reputation and which quietly kill it.

That insight helps you scale smart without burning your sender reputation.

Even with a golden source, don’t slam the gas. ISPs hate sudden spikes.

Result?

Scale to Fast…

On paper: Low CPL, massive list growth, “we’re crushing it.”

Two weeks later: Inboxing collapses, engagement falls off a cliff, revenue vanishes.

You didn’t just waste money acquiring leads—you lost the ability to monetize the list you already had.

Sustainable list growth means more than cheap leads and vanity numbers.

It’s about:

👉 The marketers who win aren’t the ones with the biggest lists. They’re the ones with the most profitable and engaged lists—because they know the real math behind subscriber value.

At the end of the day, inbox placement is the great equalizer. You can pump as much money as you want into acquisition, but if those subscribers don’t activate—and stay active—you’re just renting names, not building a business.

The brands that win are the ones that know their numbers cold: CPL, activation rate, ALE, LTV, and deliverability health.

That’s the formula for profitable growth at scale.

Chris Miquel

PS: If you’re still measuring list growth by CPL alone, you’re driving blind. Get serious about activation, deliverability, and subscriber LTV—and watch your email program stop leaking money and start compounding profit.

BEFORE YOU GO

If your emails aren’t landing in the inbox, they’re not doing their job. I’ve seen too many brands struggle with deliverability issues without knowing why.

The truth is, a few key optimizations can make all the difference in getting your emails seen, opened, and clicked.