A manager buys 500,000 shares at $42.00. Three days later, the average fill is $43.18. Commissions were $12,500. But the total implementation cost was $590,000, market impact, delay costs, and opportunity cost consumed 50x more than the explicit fees.
Commissions: fees paid to brokers. Institutional rates have declined to fractions of a cent per share for algorithmic execution. Vary by broker, venue, order complexity, and service level.
Exchange fees: maker-taker models charge a fee for taking liquidity and rebate a credit for providing liquidity. Some exchanges use inverted pricing (pay takers, charge makers).
Taxes: transaction taxes apply in some jurisdictions. UK stamp duty of 0.5% on equity purchases is material. Transaction taxes directly reduce active trading profitability.
Custody and settlement fees: small individually but accumulate with high turnover.
Bid-ask spread: the difference between the best sell price (bid) and best buy price (ask). A stock at $49.90/$50.10 has a $0.20 spread. Crossing the spread to execute immediately costs $0.10/share relative to the midpoint. Spreads widen for illiquid securities, high volatility, and larger orders.
Common mistakes
- Reporting only commissions as trading costs. Explicit costs are the tip of the iceberg. Market impact and opportunity cost typically exceed commissions by 5-10x for institutional orders.
- Using VWAP without acknowledging its limitations. VWAP captures execution quality relative to the trading day but misses the decision-to-execution gap. IS captures the full cost from decision to completion.
- Forgetting the factor of 2 in the effective spread formula. Effective spread = 2 x |execution price - midpoint|, not just the one-way deviation. The 2 makes it directly comparable to the quoted (round-trip) spread.
Bottom line
- Explicit costs (commissions, fees, taxes) are visible; implicit costs (spread, market impact, delay, opportunity cost) are far larger
- Implementation shortfall measures total cost from decision to execution (Paper portfolio return minus Actual portfolio return), the gap between the paper and the actual portfolio
- Effective spread = 2 x |execution price - midpoint|, scaled by 2 so it compares directly with the round-trip quoted spread
- Market impact increases with order size, urgency, and illiquidity; it is the dominant implicit cost for institutional trades
Exam shortcut
For IS calculations, set up the paper portfolio first (all shares at decision price), then the actual portfolio (filled shares at actual price minus commissions). The difference is total IS. Decompose by component: explicit (commissions), realized cost (filled shares x price difference from decision), and opportunity cost (unfilled shares x price movement). For algorithm selection: alpha urgency = IS algorithm; no urgency = VWAP; illiquid/no pattern = TWAP.
The full lesson (about 5,201 words, 35 min read) adds 2 worked examples, all 11 common mistakes, a self-check, free in the app.
Learning objectives
- trading costs
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