A percentage-based commission charges a proportion of the position's notional value rather than a fixed amount per lot. It is the normal structure on share and crypto CFDs, where instruments differ so widely in price that a flat per-lot figure would mean nothing, and it appears on some forex accounts as an alternative to per-lot pricing.
The two structures behave differently as the trade changes. A per-lot commission is fixed once the size is fixed, so its weight relative to the instrument's value falls as that value rises; a percentage commission scales with the value of what you bought, so it stays constant in relative terms and grows in money terms. Converting both to a single measure — cost per million of notional, or cost as a share of the instrument's typical daily range — is the only way to compare them, and the comparison should be run at the size actually traded rather than at one lot.
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Spreads, Commissions and the Real Cost of a Trade
Spread, commission and swap are the three costs on every forex trade — what each one is, when spreads widen, and how to compare pricing fairly.