An example
Picture a currency booth on the street. The board says it buys euros at 116 and sells at 118 dinars. Change euros into dinars and immediately change your mind, and you have lost two dinars per euro — without paying any “commission” at all.
Crypto works the same way, except the board is not always visible. The exchange shows one price and buys back at a slightly lower one. That gap is their earnings, on top of the commission.
Why it matters when comparing
An exchange boasting a low commission may have a wider spread than one with a higher commission. The total cost is the sum of the two, not just the number in the price list.
That is why our coin pages calculate how much crypto you actually receive for your amount, instead of only showing a percentage.
When the spread is wider
The spread widens when fewer people are trading. On bitcoin it is usually small. On small cryptocurrencies it can run to several per cent, because there are not enough buyers and sellers.
It also widens when the market is turbulent. On days of big jumps and falls, exchanges widen the gap to protect themselves.
Instant one-click buying almost always carries a wider spread than setting your own price on the exchange. That is the price of simplicity.