A bonus of 100 is not 100 in your pocket. It usually comes with a wagering requirement: you must stake an amount before you can withdraw. Terms differ. Always read them.
The maths
Every bet has an expected cost equal to the margin times the stake. A wagering requirement forces you to place many bets, so you pay the margin many times.
Expected bonus value ≈ bonus − (turnover × margin). It is an approximation: it ignores the risk of losing the balance before you finish the turnover.
Free bets
A free bet usually does not return the stake when you win. You only get the profit. If the fair probability is 1 / odds, the value is stake × (odds − 1) / odds.
What to check
- Whether the requirement counts the bonus only or the deposit too.
- Minimum odds per bet and markets that do not count.
- The deadline to finish the turnover.
- The cap on what you can withdraw from bonus winnings.
How to judge it
Before you accept a bonus, do three sums. First find the turnover. If the requirement is 8 times (deposit and bonus together) and you deposit 50 with a 50 bonus, the turnover is 8 × 100 = 800.
Then multiply by the margin you expect: 800 × 0.05 = 40. Finally subtract from the bonus: 50 − 40 = 10. The bonus is worth about 10, not 50.
If the result is near zero or negative, you lose nothing by skipping it. A wagering requirement is not a trap in itself, but it is not a gift either. Think about the time it will take, too.
Risk counts too. With a turnover of 800 and a 5% margin, the expected cost is 40, but the real result will differ from that. You can lose the balance before you finish the turnover, and then the bonus is worth nothing. The average describes many repetitions, not your single one.