How does the bid-ask spread absorb transaction costs?

Question

AcadiFi · Accepted Answer

The bid-ask spread is the difference between the best offer (ask) and the best bid. A market order to buy pays the ask; a market order to sell receives the bid. Crossing the spread once costs half the spread relative to the mid; a round trip costs the full spread.

For a trader named Anisha Chowdhury buying 10,000 shares of a small-cap where bid is $42.90 and ask is $43.10, she pays $43.10 per share. The mid is $43.00; her implicit cost is $0.10 per share — $1,000 total — before explicit commissions.

The spread compensates market makers for three things:

1. **Order processing costs** — operational and technology.
2. **Inventory risk** — carrying the position between trades.
3. **Adverse selection** — the chance the counterparty has private information.

Illiquid securities have wider spreads because market makers need more compensation, particularly for adverse selection risk.

Measurement nuances: the **quoted spread** is ask minus bid. The **effective spread** is 2 × |trade price − mid| — it captures what was actually paid, which may differ if trades occur inside the quoted spread. The **realized spread** subtracts post-trade price drift to isolate the pure market-maker profit from information costs.

Large orders face more than half-spread. Walking the book eats multiple price levels and signals information, pushing effective cost above half-spread. This is why institutional traders use VWAP algos, dark pools, and block venues — to minimize the visible spread footprint.

How does the bid-ask spread absorb transaction costs?

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