Work out what a bonus is actually worth: expected cost of clearing the playthrough at a given house edge, and whether the offer beats no bonus at all.
A wagering requirement converts a headline bonus into an expected cost. If you must turn over a bonus thirty times on games with a 4% house edge, the expected cost of clearing it exceeds the bonus itself — the offer is negative value however large the number on the banner.
This calculator does that arithmetic: bonus amount, playthrough multiple and house edge in, expected value out, so you can tell a genuine rebate from a balance-retention mechanism.
Expected cost of clearing equals bonus amount multiplied by the playthrough multiple multiplied by the house edge of the games you clear it on. Compare that against the bonus. If cost exceeds bonus, the offer is worth less than nothing, and taking it in exchange for locking your own balance is worse still.
This is also why game restrictions matter more than the bonus size. Clearing at 1% house edge versus 4% changes the answer by a factor of four.
It converts a bonus and its playthrough into an expected cost: how much of the bonus you can expect to lose to the house edge while clearing it. The comparison that matters is against zero — against simply playing without the bonus and keeping the freedom to withdraw.
Two inputs dominate the answer. The wagering multiple decides how much turnover you are committing to, and the house edge of the game you clear it on decides what that turnover costs. A 40x requirement on a 4% edge game is a very different offer from 40x on a 1% one.
Almost never on high-edge games. Run it through the calculator: the expected cost of turnover usually exceeds the bonus by a wide margin.
Typically no, which is exactly why it is worth more per dollar than a much larger locked bonus.