Market mechanics
The idea
Trading costs can enter at different moments. The spread separates bid and ask; commission may apply on entry and exit; financing may apply to positions held beyond a rollover time. A strategy with small average gains is especially sensitive to these costs. Compare like-for-like contract sizes and holding periods.
A worked example
If gross expectancy is USD 4 per trade and average round-trip costs are USD 5, the estimated net expectancy is minus USD 1.
Try it yourself
Build a sample trade record with spread, both commissions and overnight financing shown separately.
A mistake to avoid
Comparing a low advertised spread with a different product's all-in cost.