Direct answer
Costs can affect the bid-ask spread in two main ways: directly (by adding explicit charges to trading) and indirectly (by changing the risks and frictions a liquidity provider accounts for when quoting prices). Even when the displayed spread looks stable, total cost to trade can still change due to commissions, funding/financing charges, or wider quoting when conditions become riskier.
Mechanism and definition
A bid-ask spread is the difference between the bid price (what a market participant is willing to buy at) and the ask price (what they are willing to sell at). The spread matters because it is the immediate “round-trip” cost of entering and exiting, before you account for other charges.
Two cost categories are useful:
- Direct costs: explicit, measurable charges tied to executing trades (for example, dealing commissions or platform fees). These costs may not always change the displayed spread, but they can increase the effective cost of trading.
- Indirect costs: expenses that show up in how quotes are set rather than as a line item on your statement. These include the cost of holding or adjusting inventory, compensation for adverse selection (trading against better-informed orders), and risk premiums when price moves are harder to predict.
A provider sets bid and ask levels to balance profitability and risk. When the expected cost of trading rises, providers may widen spreads to protect against losses.
Evidence or example (with clear assumptions)
Assume a quote shows:
- Bid = 1.1000
- Ask = 1.1002 So the displayed spread is 0.0002 (in price terms).
Now assume there is also an explicit commission of 0.00005 per unit (written here as “cost in the same units as the price gap” for illustration). Under that assumption, the effective total cost at entry becomes 0.0002 (spread) plus 0.00005 (commission), even though the provider’s displayed spread might remain unchanged.
Next, consider an indirect cost channel. Suppose volatility rises and incoming orders are more likely to move the market quickly against the provider’s position. Even without any change in explicit fees, the provider may widen the bid-ask spread to cover higher inventory and execution risk.
In practice, you verify which channel is operating by separating what changes:
- If displayed spreads widen while commissions stay the same, indirect costs and risk pricing are likely increasing.
- If displayed spreads stay similar but reported total charges increase, direct costs are likely the main driver.
You can do this without real-time data by using your own execution records: compare the spread shown at execution time and the total all-in cost you were charged under different market conditions.
Limitations and failure modes
At least one material limitation is that spread behavior is not fixed. It can change with liquidity and volatility, and it can differ by execution venue and order type.
Common failure modes when interpreting “cost → spread” are:
- Confusing displayed spread with total cost: commissions or other charges can move total cost even if the displayed spread does not.
- Assuming past relationships persist: historical patterns between volatility and spread width do not guarantee future behavior.
- Ignoring timing: spreads can jump during fast market changes, news, or brief liquidity gaps.
Also, costs can overlap: when volatility rises, both indirect risk costs and some direct charges may increase depending on provider policies.
Verification and next question
To independently verify what costs are affecting bid-ask spread in your context, compare three things across conditions (and keep assumptions consistent):
- Displayed spread at execution (bid minus ask, recorded at the time you place/execute).
- Explicit charges shown in your trading or account records (commissions, dealing fees, and any per-trade fees).
- Condition changes (for example, periods of higher market movement), since indirect costs often rise when liquidity and price predictability worsen.
Next, ask: Does the spread change appear before or after explicit charges change? If the spread widens without changes in explicit fees, indirect costs are likely dominating. If total cost changes while displayed spread remains similar, direct costs are likely dominating.