Robo-advisor marketing tends to emphasize low fees, but "low" only means something once you understand which fee model is being used. This guide breaks down the three common structures, building on the broader framework for evaluating a robo-advisor.
The Three Common Fee Models
Most robo-advisors use one of three approaches:
- Percentage of assets under management (AUM) — a yearly percentage of your invested balance, often billed monthly or quarterly.
- Flat fee — a fixed dollar amount per month or year, independent of balance.
- Subscription — a fixed recurring charge, sometimes tiered by feature access rather than balance.
Why AUM Fees Scale With Your Balance
An AUM fee looks small as a percentage, but the dollar cost grows every year your balance grows. A fee that costs relatively little on a small starting balance can become a much larger annual dollar amount once decades of growth and contributions have built up a larger account — the percentage never changes, but what it's a percentage of keeps increasing.
Why Flat Fees Behave Differently
A flat fee stays constant in dollar terms no matter how large your balance becomes. This tends to make flat fees relatively cheaper for investors with larger balances and relatively more expensive, as a percentage of a small balance, for those just starting out.
| Balance size | AUM fee (dollar cost) | Flat fee (dollar cost) |
|---|---|---|
| Smaller balance | Lower dollar cost | Can be proportionally higher |
| Larger balance | Higher dollar cost | Stays the same, proportionally lower |
Subscription Models and What They Include
Subscription pricing detaches cost from balance entirely, instead charging a fixed periodic fee — sometimes with tiers that unlock features like tax-loss harvesting or access to a human planner. This can make costs predictable, but it is worth confirming exactly which features are included at each tier before assuming a lower-tier subscription covers everything you want.
Why Fee Drag Compounds
This is why comparing fee structures matters more the longer your investing time horizon is. A short-term account is less sensitive to fee differences than a retirement account held for decades.
Don't Forget Fund-Level Costs
The advisory fee is usually separate from the expense ratios charged by the underlying ETFs or index funds a robo-advisor invests in. A platform with a low advisory fee but expensive underlying funds may not actually be the cheapest option once both costs are combined.
How to Compare Fairly
- Estimate your expected account balance over your investing time horizon.
- Calculate the total annual dollar cost under each fee model at that balance.
- Add any separate fund-level expense ratios to get your true all-in annual cost.
- Weigh that total cost against the features — like automatic rebalancing — you're actually getting for it.
Common Mistakes to Avoid
- Comparing percentage fees without projecting the dollar cost at your expected balance.
- Ignoring fund-level expense ratios layered on top of the advisory fee.
- Assuming a flat fee is always cheaper without checking your own balance size.
- Choosing a subscription tier without confirming which features are actually included.
Conclusion
Fee structure shapes cost differently depending on your balance and time horizon — there is no universally cheapest model. Project your own numbers under each structure, add in fund-level costs, and weigh the total against the features that matter for your specific evaluation of a robo-advisor.