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The value of each pair differs slightly but the minimum movement - called a pip spread - is worth approximately $10. The GBPUSD has been averaging 100-150 pips per day.
Brokers tend not to charge a commission for trading forex and you will often see adverts for "commission free" trading. However, they make their money on the pip spread which is the difference between the buying price and the selling price. The pip spread is usually between 3 and 5 pips spreads although some brokers may offer a 1 pip spread on some pairs, and some less-popular pairs may have a larger pip spread.