Why Founders Should Pay More Attention to Customer Acquisition Cost

We are increasingly seeing headlines about startups reaching millions in Annual Recurring Revenue (ARR) within months of launching or about ARR accelerating month after month. Below are the case in point - screenshots of headlines taken from TechCrunch.

^{Screenshots were taken from TechCrunch}

Undeniably, building a startup has become easier in many ways, especially with AI reducing the time and resources required to develop products and services. Software startups are an obvious example, given how much of the development process can now be accelerated with AI tools.

Sure, the barriers to building a startup have come down, but I’m more interested in the headlines themselves, especially the numbers in them. An increase in ARR or MRR is good news for sure, but what about the unit economics? I’m referring to metrics such as Customer Acquisition Cost (CAC), Customer Lifetime Value (LTV), the LTV to CAC ratio, CAC Payback Period and Contribution Margin. Not exactly the sort of numbers that would have everyone rushing to click on an article.

Imagine coming across a headline that reads, “ABC Tech spent three times less to acquire five times more customers.” Interesting? Maybe, especially if you’re a founder, investor or someone who gets unusually excited about financial metrics. But compare that with a headline about a startup reaching $10 million in ARR within six months. You’d probably read it. I’d read it too. Simple, no?

Here’s where I think more attention is warranted. We know how much revenue these startups are generating, yet we often know very little about what it costs them to generate that revenue. A startup reaching $10 million in ARR within 6 months is impressive, but would you still be impressed if you knew they had spent an unsustainable amount of money acquiring those customers? Yeah, thought so! That’s why I believe metrics like the customer acquisition cost should come into the picture, and why founders should pay more attention beyond the headline numbers.

Let’s sort the basics out first.

What is Customer Acquisition Cost?

Customer Acquisition Cost (CAC) measures how much a company spends on acquiring a new customer. Simply put, CAC involves the costs and resources needed to acquire an additional customer. ¹ The calculation is quite straightforward - take the total amount spent on acquiring customers over a certain period and divide it by the number of new customers acquired.

CAC = Total Sales and Marketing Expenses ÷ Number of New Customers Acquired ²

Let’s work with some numbers. Suppose ABC Tech spent $50,000 on sales and marketing over three months and acquired 500 paying customers. Input these values in the formula and you’d get a CAC of $100. Plugging in the numbers sure is straightforward but the difficulty lies in deciding the variables that belong in that $50,000 spend because it sure includes far more than Google and Meta ads. Salaries, commissions, software subscriptions and fees all contribute to winning customers, and leaving them out will make your CAC look way better than it truly is.

Timing matters too because some startups especially enterprise SaaS company may spend months on demos, negotiations, and procurement before the founder sees money flowing in from customers. That simply means matching one month’s spend to that same month’s new customers can give a misleading picture. So the CAC does matter. It always did. But why?

Why Does CAC Matter?

Let’s take two startups as examples - ABC Tech and XYZ Tech, operating in the same industry with similar subscription products. Each has successfully acquired 1,000 paying customers over the past three months, with ABC Tech spending $20,000 and XYZ Tech spending $100,000. Both could announce 1,000 customers and be perfectly accurate, even though one paid five times more to get there. If we assume both charge $20 a month at a 50% gross margin, each customer brings in $10 of gross profit a month, and the difference becomes much easier to see.

That last row is the CAC payback period, which is how long a company takes to recoup its acquisition spend through the gross profit those customers generate. ABC Tech needs about two months while XYZ Tech needs ten, and the difference is important because the money has to come from somewhere, whether that is internal cash, an earlier funding round or debt. A longer payback keeps capital tied up for longer, so if both companies wanted another 5,000 customers at the same CAC, ABC Tech would need $100,000 while XYZ Tech would need $500,000.

That said, a lower CAC is not automatically a better one, which is where Customer Lifetime Value (LTV) comes in ³ Imagine ABC Tech wins customers through paid advertising at $30 each, and they stay four months at $10 of monthly gross profit, while referral customers cost $80 each and stay two years. The paid channel looks cheaper but returns only $40 in lifetime gross profit, or about $1.33 for every $1 spent, whereas referrals return $240, or $3 for every $1 spent. This is the LTV to CAC ratio, and it explains why the cheapest channel is not always the best one, although I would add that LTV is an estimate and a rather uncertain one for early stage startups. A spreadsheet can happily project five years of retention from eight months of data, but customers are under no obligation to follow it.

CAC is also not fixed, which many founders overlook. The first 1,000 customers often come from founder networks, communities and referrals, whereas the next 10,000 may need paid channels, bigger sales teams and new markets. If ABC Tech won its first customers at $20 each and projected the next 10,000 at the same price, it would budget $200,000, but if paid channels push the cost to $60 each, the real bill is $600,000. Looking at the marginal CAC ⁴ of the next batch of customers, rather than the historical average, gives a far more honest view of what growth will cost.

The Cost Behind the ARR Headline

ARR and MRR show how much recurring revenue a company has, but they say very little about how efficiently it acquired the customers behind that revenue. Two startups could both report $10 million in ARR, with one enjoying excellent acquisition economics and the other spending heavily on customers who leave before the business has recovered what it spent winning them. The headline would look identical while the businesses underneath could hardly be more different.

When a founder shows me a deck where ARR grew from $1 million to $5 million in twelve months, it certainly gets my attention, but I immediately want to know how much went into sales and marketing, what retention looks like and how much more capital it would take to reach $10 million. Spending ahead of revenue can be a perfectly deliberate strategy when the long term economics justify it, though there is a real difference between investing in an efficient acquisition engine and spending heavily because no cheaper route to growth has been found.

This matters just as much if you are the one building. Measuring your progress against a headline, without knowing what it cost the company behind it, can push you into spending aggressively to keep pace with a number that may not be as healthy as it looks.

You do not need the lowest CAC in your industry. What you need is to understand how much a paying customer costs you, how that varies across channels, how long it takes to earn the money back and whether those economics are likely to hold as you scale. Reaching $10 million in ARR is impressive, but being able to explain what it cost, and whether you can repeat it, tells us far more about the business, and perhaps that is a headline worth paying attention to.