Transaction costs are subtracted from returns silently. Unlike a management fee, they do not appear as a line item — they appear as a slightly worse entry price, which is indistinguishable from ordinary market movement unless it is measured deliberately.
The components
Commission is the visible one, and usually the smallest.
Bid-ask spread is the difference between the best offer to buy and the best offer to sell. Every trade crosses it. In a heavily traded instrument it is tiny; in a thinly traded one it can be several percent.
Market impact is the movement caused by the trade itself. A large order consumes the available liquidity and pushes the price against the buyer. Impact grows with size and shrinks with liquidity, and it is the reason a position that looks profitable on paper may not be achievable in practice.
Timing and opportunity cost — the price moved while the order was being worked. Unavoidable in any market that is moving.
Taxes where they apply, which for a taxable investor can exceed all the others combined.
Foreign exchange if the instrument trades in a different currency.
Why this matters more than it looks
Costs are recurring, so they compound against the portfolio the same way returns compound for it. A strategy that trades frequently pays them frequently.
The most important consequence is not the average cost but the capacity limit. A strategy that works at small size may stop working at large size, because the trading itself moves the price. This is a genuine constraint on how much capital a strategy can absorb, and it is often the reason a good idea cannot be scaled.
Liquidity, properly understood
Liquidity is not a property of an instrument; it is a property of the market at a moment. The same bond may be easy to trade on a calm Tuesday and impossible to trade during a credit event.
Three questions are worth asking before committing capital to anything illiquid:
- Who would buy this if I needed to sell in a hurry? If the answer is "the same people who sold it to me, when they feel like it," that is a risk.
- How long would it take to exit at a fair price? Days, months, or only in a functioning market.
- What happens to the price if several holders need to exit at once? For an asset held widely, this is a real scenario, not a hypothetical one.
Measuring rather than assuming
The measurement is straightforward in principle: compare the price at the moment the decision was made with the actual executed price. The difference is the cost.
Doing it matters for three reasons:
- It reveals whether a strategy's apparent return survives its own trading. A backtest that ignores costs is not evidence.
- It shows where costs concentrate. Often a small number of large or illiquid trades account for most of them.
- It makes the trade-off explicit. Trading more frequently may improve the position in theory and worsen it in practice.
Eonvera's research incorporates liquidity assessment and transaction costs into investment analysis rather than treating them as an operational afterthought — because a return that cannot be realised after costs is not a return.