Two different kinds of spending
Paid search is an operating cost with a linear relationship to volume: double the clicks, double the spend, and stop paying and traffic goes to zero within hours. That predictability is a genuine feature. It is the only channel you can turn on this quarter, test in a fortnight and switch off when cash is tight, and it produces conversion data on commercial intent faster than any keyword tool.
SEO behaves like capital expenditure. The cost is front-loaded and largely independent of volume: the same technical work and the same page serve a hundred visitors or a hundred thousand. Once earned, positions decay slowly rather than disappearing, which is why the model lets you set a residual — the share of traffic organic still delivers if the programme stops. In most portfolios that residual is between 40 and 70 % a year after work ends, and it is the single biggest reason the two channels are not comparable on cost per click alone.
The consequence is that the honest comparison is not "which is cheaper" but "which is cheaper for how long, and what does each buy besides traffic". Paid buys certainty and speed. Organic buys a declining cost per acquisition and an asset that keeps working during the months your budget gets frozen.
What the crossover month really tells you
The crossover is where cumulative cost per acquisition for organic falls below paid. On typical inputs — a competitive CPC, a nine-month ramp, a mid-sized retainer — it lands somewhere between month twelve and month twenty. Two things move it more than anything else: the ramp, and CPC inflation. A market where clicks get 15 % more expensive every year brings the crossover forward by months without SEO doing anything differently.
If the crossover never arrives within your horizon, that is a legitimate result, not a broken model. It usually means one of three things: the CPC is genuinely low and paid is simply efficient, the SEO retainer is too high for the traffic it targets, or the ramp assumption is too pessimistic for a site that already has authority. Change one input at a time and watch which one is carrying the conclusion.
Beware of using the crossover to justify cutting paid the moment organic overtakes it. In most accounts the two channels are not fully substitutable: paid covers queries organic will never win, protects the brand term against competitors, and carries promotions that a static page cannot. The healthiest reading of a crossover is "organic can now fund its own growth", not "switch the ads off".
- Compare at equal click volume, not at equal budget
- Enter separate conversion rates for paid and organic
- Model CPC inflation over any horizon longer than a year
- Set the residual honestly: organic decays, it does not vanish
- Keep paid on brand terms and promotions regardless of the crossover
- Re-run the comparison annually as both costs move
Running both without wasting either
The teams that get the most out of this comparison use paid as reconnaissance. Run ads on the commercial keyword clusters you are considering, measure which ones actually convert, and only then commission the pages. It costs a few thousand and removes the largest risk in any content plan: writing well for an intent that does not buy.
In the other direction, organic data should shape the paid account. Queries where you already rank first and hold a high CTR are usually the worst places to keep bidding at full budget; queries where organic is stuck on page two and the intent is transactional are where paid earns its margin. This kind of arbitrage needs both datasets in one report, which is the practical argument for the reporting discipline described in our KPI guide.
If you are buying either service, the same due diligence applies to both: ask for measured outcomes, not screenshots. Agencies listed here that connect their Search Console publish verified performance, and the directory also lists specialists in paid search if you need the two disciplines under one roof.