How the two channels differ
Both channels put you in front of people who are searching. The economics are what differ.
| Factor | Paid search | SEO |
|---|---|---|
| Time to first traffic | Days | Usually months |
| How you pay | Per click, at auction prices | For work: content, fixes, links |
| When spend stops | Traffic stops | Traffic fades gradually |
| Cost per sale over time | Tracks click prices | High early, tends to fall as traffic grows |
| Control and testing | High, fast feedback | Lower, slower feedback |
| Main risk | Rising click prices | Slower or smaller gains than planned |
Neither is better in general. Each is better for a particular job. The way to decide is to put both on the same measure over the same period.
Compare on cost per sale
The fairest common measure is cost per sale, or cost per qualified lead for B2B. It is simple: total channel spend ÷ sales from that channel. Avoid comparing on ROAS (revenue ÷ ad spend). It ignores margin, and there is no clean SEO equivalent.
Cost per sale = channel spend ÷ sales from the channel
Count the full cost on both sides. For paid search, that means click spend plus agency or management fees, landing page work and tools. For SEO, it means fees, content, links, tools and the internal hours of writers and developers. Leaving management costs off the paid side is as misleading as leaving staff time off the SEO side.
The window matters a great deal. Over one quarter, paid ads will almost always win, because SEO has barely started. Over 24 months the picture can change. Pick the window before you look at the result, and use the same one for both channels.
Worked example over 24 months
A shop has $6,000 a month to spend and is choosing between the two channels. The figures are illustrative assumptions, not benchmarks. Click prices and conversion rates vary widely by market, so swap in your own.
Paid search
At an assumed $1.50 per click, $6,000 buys 4,000 clicks a month. At a 2% conversion rate that is 80 sales a month. Cost per sale: $6,000 ÷ 80 = $75. Over 24 months: 1,920 sales for $144,000, still $75 a sale, as long as click prices hold.
SEO
The same $6,000 a month goes to content and technical work. Assume sales from new organic traffic ramp up: an average of 10 a month in months 1 to 6, 50 a month in months 7 to 12, and 120 a month in year two.
| Period | Spend | SEO sales | SEO cost per sale | Paid cost per sale |
|---|---|---|---|---|
| Year 1 | $72,000 | 60 + 300 = 360 | $200 | $75 |
| Year 2 | $72,000 | 1,440 | $50 | $75 |
| Both years | $144,000 | 1,800 | $80 | $75 |
Read it carefully. In year one, SEO costs $200 a sale against $75 for paid. In year two, SEO costs $50 against $75. Across both years they are close: $80 against $75. On these inputs, paid search is still slightly cheaper over 24 months.
So why choose SEO at all? Because the two lines are heading in different directions. At the end of year two, SEO is producing sales at $50 each and the traffic does not vanish if a month's budget is cut. Paid is producing sales at $75 for as long as you keep paying, and that price rises if competitors bid up your keywords. A third year usually decides it. That is an assumption to test, not a promise, so model it with the SEO vs PPC comparison tool.
Change one input and the answer moves. If SEO only reached 90 sales a month in year two, its two-year cost per sale would be $144,000 ÷ (360 + 1,080) = $100. Always show the cautious case next to the expected one.
When paid ads make more sense
- You need sales this quarter. Cash flow or a launch date cannot wait for SEO to ramp.
- You are testing demand. Paid shows within weeks whether people search for and buy an offer.
- The offer is short-lived. Events, seasonal stock and promotions end before SEO would pay back.
- Organic results are crowded out. Some searches show so many ads and features that organic clicks are scarce. Check your Search Console CTR on those terms.
When SEO makes more sense
- You can wait 12 to 24 months. The longer the horizon, the more SEO's falling cost per sale counts.
- Click prices are high. When each click is expensive, organic visits are worth more. The organic traffic value tool estimates what your current visits would cost to buy.
- Buyers research before they buy. Guides and comparison pages answer the questions ads cannot.
- You want an asset, not a rental. Pages that rank keep working for a while after the budget slows.
When to run both
Most businesses with a real budget run both, with each covering the other's weakness.
- Use paid while SEO ramps. Cover high-intent terms with ads in year one, then reduce bids as organic positions settle.
- Use paid data to pick SEO targets. Search terms that convert well in ads are strong candidates for new organic content.
- Test overlap, do not assume it. Where you already rank first organically, pause ads on some terms and keep a comparison group. Check the result with the significance checker before you cut spend everywhere.
- Report both on the same measure. Cost per sale by channel, same attribution model, same window. Reporting set up this way makes the next budget decision much easier.
A common split is to set a paid budget that covers the sales target for the next two quarters, and an SEO budget sized by its own payback month. Review both every quarter. As organic cost per sale falls below paid on a group of keywords, move budget across in steps, not all at once, and watch total sales rather than channel totals alone.
If you want to see how SEO alone stacks up on your numbers, start with the SEO ROI calculator. Then read the payback guide to find the month SEO covers its cost.
Questions people ask
Is SEO cheaper than paid ads?
Not at first. SEO usually costs more per sale in the early months because you pay before traffic arrives. Over a longer window, cost per sale from SEO often falls while paid stays tied to click prices. Model both on your own numbers to see where they cross.
Can I stop paid ads once SEO is working?
Sometimes, for specific keywords. Test it rather than assume it: pause ads on a set of terms where you rank well organically, keep a comparison group running, and check whether total sales hold.
Should I compare SEO and PPC on ROAS?
ROAS uses revenue and ignores margin, so it can make both channels look better than they are. Compare cost per sale or profit-based ROI instead, over the same time window.
Does SEO traffic stop when I stop paying?
Not immediately, unlike ads. But rankings tend to fade if competitors keep working and you do not. Do not assume organic traffic stays flat forever after spending stops.