Commissions and Slippage Against a Small Edge
Costs are the least interesting part of a trading strategy and one of the few parts you can actually control. They are also the reason a great many methods that look viable on paper are not viable in an account. The mechanism is simple and unforgiving. Every cost is deducted from the same average result the strategy is trying to generate, and it is deducted on every trade regardless of how the trade went.
Costs Are Not a Separate Line Item
It is tempting to think of commissions as overheads, paid out of profits at the end of a period. That framing hides what is happening. Each cost reduces the result of the individual trade it belongs to, which reduces the average win, increases the average loss, and lowers the expectancy figure directly.
The consequence is that costs damage a losing trade as well as a winning one. A strategy with a low win rate pays them on every failed attempt, and those payments accumulate through exactly the stretches where the account can least tolerate them.
Three Different Costs, Three Different Behaviours
Commissions and exchange fees are the visible portion and the easiest to model, because they are known in advance and generally fixed per unit traded. The spread is the second, paid at both ends, and it widens at precisely the moments an opening range strategy is active, since the first minutes of a session are frequently the least orderly.
Slippage is the third and the one that resists measurement. It is the difference between the price you intended and the price you received, and it is systematically worse for breakout entries than for most other approaches, because a breakout entry is by construction an order placed into a market that is already moving in that direction. Stop orders sitting at obvious levels tend to fill worst on the sessions that move most.
Why a Small Edge Suffers Disproportionately
Costs are close to fixed per trade while the edge is a small residue. That relationship is the whole problem. A method whose average result per trade is comfortably large treats transaction costs as a minor deduction. A method whose average is slim can find that the same absolute deduction consumes a substantial share of it.
Two further factors make it worse. Frequency multiplies the cost without necessarily multiplying the edge, so a strategy that takes several attempts per session pays several times over for a single day's opportunity. And the smaller the intended move, the larger costs loom relative to it, which is why scalping variants of a breakout are far more cost sensitive than versions holding for a larger portion of the session.
Measure Them From Fills, Not From Assumptions
The commission schedule is published and the spread is observable, so both can be estimated reasonably well in advance. Slippage cannot, and the only credible source is your own fills. Recording the intended price alongside the received price on every trade builds that dataset over time and turns a guess into a measurement.
What usually emerges is that slippage is not evenly distributed. It concentrates in identifiable circumstances, wide ranges, the first minutes after the open, sessions following an overnight development. Knowing where it clusters is more actionable than knowing its average, because a cluster can sometimes be avoided while an average can only be accepted.
What Can Actually Be Reduced
Commission rates are negotiable at some brokers and vary considerably between them, and for a frequent strategy that difference alone can matter more than most changes to the entry rules. Trading fewer, better qualified setups reduces total cost directly and is usually available without any change to the method beyond enforcing conditions already written down.
Order type is the other lever. An entry that can be placed as a resting order rather than crossing the spread avoids part of the cost, though it introduces the risk of not being filled on the moves that run without pausing. That is a genuine trade off rather than a free improvement, and which side of it is correct depends on how often your strategy's winners come from the fast breaks.
The habit worth building is simple. Whenever expectancy is calculated, calculate it net, and never quote the gross figure even privately. The gross number describes a strategy nobody is able to trade, and the difference between the two is the part that decides whether the method belongs in an account.