The inevitable decline of return on ad spend as budgets scale
Marginal return on ad spend follows a natural decay curve. Your first $1,000 of spend might generate $5 in revenue (5x ROAS). Your next $1,000 might generate $4 (4x), then your third might generate $3 (3x). This happens because you're pulling demand from highest-intent pools first. The person actively searching for your product converts easier than the person in a consideration phase who sees a remarketing ad. As you scale spend, you exhaust high-intent pools and move to lower-intent audiences with lower conversion rates.
The curve is not a bug; it's the natural shape of marketing efficiency. The goal is finding your efficient frontier, the spend level where marginal ROAS still covers your cost of goods sold plus a target margin. If your goods cost $20 and you want 100% margin, you need 1.33x ROAS minimum (if 1 sale = $20 spend to break even). If you find a sweet spot at 2.5x ROAS, you're pulling 100% profit. But at 1.5x ROAS, you're only pulling 50% profit per sale. Scale past that point, and you're losing money on marginal spend.
Identifying and testing your saturation point
The frontier is product-specific and changes monthly. High-ticket items have less saturation faster because the addressable market is smaller. Commodity products saturate quickly because you're competing in a large pool. The way to find your frontier is to double spend and measure what happens to ROAS. If you go from $1,000 to $2,000 weekly spend and ROAS drops from 4x to 3x, you're moving down the curve. If it drops from 4x to 1x, you've hit saturation hard and should hold spend at the previous level.
Once you've found the efficient frontier for one channel (Google Search at 2.5x ROAS, say), don't grow that channel further. Instead, expand to adjacent channels (Google Display, YouTube, Meta) which have their own demand curves. Each channel's users have different intent profiles and saturation points. A diversified portfolio across channels lets you grow total revenue while maintaining targeted ROAS on each channel. Scale overall revenue by adding channels, not by pushing single channels past their efficient frontier.