Separate the components
List commissions, exchange or regulatory fees where applicable, spreads and slippage. Borrowing or financing costs require separate treatment when relevant. A single flat number may be a useful approximation, but label the approximation.
Apply costs to transactions
A buy and a later sell are two transaction legs. In a simplified notional model, round-trip cost is buy notional times its cost rate plus sell notional times its cost rate. Different prices can produce different notionals.
Check sensitivity
Re-run the same rules with more conservative cost assumptions and inspect changes in turnover-heavy periods. Slippage is not constant across orders or markets. The calculator on this site uses your inputs and does not estimate live liquidity.
Sources & references
References support specific definitions or methodological context. Examples and research checklists are explanatory; no performance result is claimed.