Percentage of ad spend is the most common way agencies charge for paid media. It is also the model with the weakest incentives for the brand paying. This post explains why, shows the maths and says when a percentage fee is fine.
How the model works
The agency charges a fee equal to a percentage of your monthly ad spend, typically 10 to 20%. If you spend $20,000, a 15% fee is $3,000. If you spend $60,000, the fee is $9,000. The work to manage the account does not triple. The invoice does.
| Monthly ad spend | Fee at 15% | Fee as a flat retainer |
|---|---|---|
| $10,000 | $1,500 | $1,850 |
| $20,000 | $3,000 | $1,850 |
| $60,000 | $9,000 | $1,850 |
| $100,000 | $15,000 | Custom, scoped to workload |
The incentive problem
Under a percentage model, the agency earns more when you spend more. That is true whether or not your returns improve.
- Pressure to scale. The easy way to grow the agency's revenue is to raise your budget, not to fix your funnel.
- No reward for efficiency. If the team finds $5,000 of waste and cuts it, the agency's fee falls by $750. Few agencies lead with that recommendation.
- Platform ROAS bias. Higher reported ROAS justifies higher spend. Agencies on this model have a reason to prefer attribution settings that look good.
- Hidden markups. Some agencies also take a margin on media bought through their own accounts. Ask who the account owner is.
What it costs you in practice
Consider a brand with a nCAC ceiling of $45 and a margin that supports 3.0x MER. At $30,000 per month spend, the marginal dollar may return 1.8x. A good adviser would say: hold at $30,000 and fix creative. A percentage-fee agency has a conflict. Each additional $10,000 you spend adds $1,500 to their revenue.
Over a year, if a brand spends $10,000 per month more than it should, a 15% fee earns the agency $18,000 extra, while the brand loses money on that incremental spend.
What a flat retainer changes
A flat monthly fee ties the agency's revenue to the workload, not the spend. That removes the conflict.
- Advice to hold or cut budget costs the agency nothing.
- Scale decisions follow nCAC and MER, not the invoice.
- Budgeting is simple. You know your fee in advance.
- The agency invests in efficiency work such as tracking and creative because that is where results come from.
A flat fee is not automatically better. It needs a clear scope and a rolling term. Ask what is included, how many creatives you get and what happens if your spend triples.
When a percentage fee is acceptable
- Very large budgets with a tiered rate card. For example, 15% to $50,000, 10% to $200,000 and a fixed cap above. A cap limits the conflict.
- Pure media buying at scale. Programmatic or broadcast-style buying where workload really does rise with spend.
- Hybrid models. A base retainer plus a performance bonus tied to a metric you set, such as nCAC under target. These need careful definitions to avoid gaming.
Questions to ask any agency about fees
- Does your fee change if I double my ad spend?
- Do you earn anything else on my spend, such as media markup or platform rebates?
- Who owns the ad accounts, pixels and data?
- What is the contract term and notice period?
- Which metrics do you report: platform ROAS, MER, nCAC, margin?
- Would you ever tell me to cut my budget? Can you show an example?
A 12-month comparison
Take a brand that grows from $10,000 to $30,000 in monthly spend over a year, in even steps. Average spend is $20,000 per month. Growth in this range is exactly when a percentage fee climbs fastest while the agency's workload rises far more slowly.
| Model | Average monthly fee | 12-month fee |
|---|---|---|
| 15% of spend | $3,000 | $36,000 |
| Flat retainer ($1,850 on Growth) | $1,850 | $22,200 |
The difference, $13,800 in this example, buys more creative, more testing or simply a larger share of margin. It is a simplified illustration and your numbers will differ, but the shape holds: the faster you grow, the more a percentage model costs. Ask any agency to run this calculation with your own budget plan before you sign.
Does a flat fee remove the incentive to perform?
It changes it. The agency's incentive becomes keeping the client, which depends on results being real and reporting being honest. That is why the term matters. A rolling monthly agreement with 30-day cancellation gives you a clean exit if the numbers do not hold up. A 12-month lock-in removes that discipline, whatever the fee model.
Our approach
Adslark charges flat monthly retainers starting at $950. The fee does not move with your spend. You own the accounts and data, there is a 90-day sprint and then monthly rolling with 30-day cancellation. See the full structure on the pricing page, or ask for a free ad audit and judge our advice before you commit.