Where should your next thousand euros go? Thinking in marginal return

The question is not which campaign has the best ROAS, but where your next euro returns the most. That is marginal thinking, and it changes how you allocate budget across campaigns, channels and markets. Here is how to apply it.

The most important question in allocating your budget is not which campaign has the best ROAS, but where your next euro returns the most extra. That is marginal thinking, and it is a completely different question from looking at the average. Your winning campaign can have a beautiful average ROAS and at the same time have hit its ceiling, so that every extra euro there adds almost nothing. This article explains how to steer on marginal return across your campaigns, channels and markets.

What is the difference between average and marginal return?

Average return is what all your spent euros together have produced. Marginal return is what your very next euro will produce. Those two diverge the moment a campaign begins to saturate. Say your campaign runs a strong average ROAS at its current budget. That number is a look back at all the euros you already put in, including the first, cheapest conversions. The next euro, however, has to buy a more expensive, harder-to-reach customer, so it returns less than the average suggests. If you steer on the average, you keep topping up a campaign that is at its peak. If you steer on the marginal, you see exactly when the growth is gone.

Why is marginal return the right lever?

When scaling you want to buy growth, not reassurance. A high average ROAS feels good, but it does not tell you whether you can still grow. Marginal return does. As long as your next euro returns more than it costs, there is room to scale. The moment that euro returns less than it costs, you are buying loss instead of growth, no matter how nice your average still looks. So the question you keep asking yourself is simple: if I had a thousand euros extra right now, where would it return the most? That answer is almost never "just more into my best campaign", because that best campaign is often exactly the one closest to its ceiling.

• Average ROAS looks back at all euros together, marginal return looks forward at your next euro.
• A campaign that saturates keeps a good average but returns less on every extra euro.
• You buy growth where the next euro returns the most, not where the average is highest.
• The moment your marginal euro returns less than it costs, you are buying loss, not growth.

How do you allocate across campaigns, channels and markets?

Marginal thinking works at every level. Within your account you ask which campaign adds the most on the next euro. Often that is not your biggest winner, but a campaign that is not yet saturated and therefore still has room. At the channel level you ask the same: does an extra euro on Meta return more than on Google, given where both stand now? And at the market level you ask whether your next euro is better going to your saturated home market or to a new market where the first euros are actually cheap. The answer shifts constantly, because every euro you top up somewhere pushes that part closer to its ceiling. So marginal allocation is not a one-off decision but an ongoing question.

The best campaign is rarely where your next euro returns the most.

Why overinvesting in your winner slows your growth

The natural reflex is to keep pumping money into what works best. That campaign has the highest ROAS, so more budget seems logical. But every audience has a ceiling. Above a certain budget you no longer buy new customers, you buy frequency: you show the same ad more often to the same people. Your average ROAS slides slowly, but because it still looks okay, you keep topping up. Meanwhile you leave cheaper growth elsewhere untouched. Whoever thinks marginally sees that ceiling coming and shifts the budget to the place where the next euro does buy growth, whether that is another campaign, a second channel or a new market. That way you keep growing instead of saturating your winner.

How do you apply this practically?

You do not have to build complicated formulas to think marginally. It starts with asking the right question at every budget increase: not "where is my ROAS highest", but "where does my next euro return the most". Look at what happens when you give a campaign more budget: if the return holds, there is room, if it drops right away, it is at its ceiling. Raise in small steps so you can see the effect before you go too far. And always keep part of your budget ready to test new sources, because the cheapest growth often sits in a campaign, channel or market you have not fully tapped yet. We guide brands scaling from 15 to 20K toward 150 to 200K per month, and the common thread is always the same: growth comes from where the next euro returns the most, not from ever more into the same winner.

The beauty of thinking this way is that it guards you against the two biggest mistakes in scaling. The first is overinvesting in your winner until it saturates and your average falls apart. The second is giving up too early on a new source because it does not have a nice average ROAS yet, while its marginal return is actually the highest. Marginal thinking keeps you exactly in the middle: you keep scaling where it pays and shift in time to where the next growth sits.

Conclusion

Where your next thousand euros should go is not decided by your average ROAS but by your marginal return: where does that next euro return the most? That question keeps you from overinvesting in a saturated winner and points you to the cheaper growth in other campaigns, channels and markets. Want to set up your budget allocation so every euro goes to the place where it buys the most growth? Book a call and we will gladly look at it with you.