Te doy un ejemplo de formula, la formula no es la misma en cada broker claro, pero por norma general posiciones largas pagas y cortas cobras
Rollover Calculation:
When the client keeps an open position at the close of the market, he pays or receives daily interest financial rates. The principle is:
Long position: the customer pays. The formula is: Volume x point value x contract value x (Libor + 3%) / 360
Short position: the customer receives. The formula is: Volume x point value x contract value x (Libor - 3%) / 360
Note: The Rollover or finance charge is applicable to contracts cash indexes, currencies (Forex), equities and commodities. In other words, only the Futures indexes are not subject to rollover.
Saludos