Headline fees
Some venues charge a percentage of net winnings (e.g. 3–7%); others charge near-zero and earn from the spread. Always read the fee against the payout, not the stake.
Liquidity and slippage
On a deep market your order fills at the quoted price. On a thin one, a large order walks the book and you pay progressively worse prices — that’s slippage, and it’s often a bigger cost than the fee.