The spread can be calculated by subtracting the sell price from the buy price of the instrument, and can change whilst your position is open. Plus500 is compensated for its services through the "market spread". For example, when trading EUR/USD, if the buy rate is 1.3128 and the sell rate would be 1.3126, and the market spread would be 2 pips. While some instruments have a fixed spread, others have a dynamic spread, which is constantly adjusted according to the market spread.
Plus500 does not provide its products and services to residents of United States.