How a bonus buy works
Instead of waiting for the scatters to land, you pay a fixed price, from a few times to a couple of hundred times your bet, and the bonus round starts at once. The price is set so a bought round returns about the same as normal play.
Bonus rounds swing a lot: one buy can pay far more than its price, or much less. Pick a buy size your Credits can carry, and set a loss limit before you start.
Questions
Is buying the bonus worth it?
A bought round returns about the same as normal play over time, so it is a question of taste: you get the feature straight away, with bigger swings.
How much does a bonus buy cost?
It depends on the slot and your bet. The price is shown on the buy button and in a confirmation before you pay.