When you make a Bitcoin transaction, you pay a transaction fee. It is not fixed, but depends on the available capacity of the payment system. If the number of people willing to pay exceeds the capacity of the system, the price rises. Those who are willing to pay more must pay first. Transaction fees are calculated based on the dataset that makes up the transaction, not on its value. This means that percentage fees tend to be more expensive for small transactions in terms of cost than for larger ones.
