A bonus buy, or feature buy, lets you pay a fixed multiple of your stake to jump straight into the free-spins round. A typical price is 100x the bet, so a $1 stake costs $100 to buy in.
It is not a shortcut to value
The price is set so that the bonus round's average return roughly matches what you paid, minus the house edge: the published buy-RTP is usually similar to, and sometimes lower than, the base-game RTP. You are not buying an edge; you are buying variance. You compress a long session's worth of risk into one expensive spin.
The arithmetic of one buy
Take a game that quotes a buy price of 100× and a buy-feature RTP of 96.5%. The expected return on the purchase is 96.5× the stake, so the expected cost of pressing the button is 3.5× the stake — every time, regardless of what the last one paid. The distribution around that average is the part the price hides: in most bonus rounds the total comes back below the 100× you paid, and the average is only rescued by the rare round that pays several hundred times. Ten buys at a £1 stake is £1,000 committed, an expected loss of about £35, and a realistic chance that none of the ten covers its cost.
Compare the slow route. If the same feature triggers naturally about once in 150 spins, waiting for it costs roughly 150× in stakes — but those spins pay out at the base-game return along the way, so the net cost of arriving at the bonus is a fraction of that. The buy is not cheaper; it is faster, and speed is what it charges for.
Why it is watched closely
Because a single buy can be a large multiple of a normal bet, feature buys can accelerate losses fast, and several regulators have restricted or banned them for exactly that reason. In Great Britain the major studios withdrew feature buys from their UK-facing versions in 2019 under pressure from the regulator, so a buy button on a site licensed for Britain is itself a detail worth noticing. Some other jurisdictions require the feature to be disabled entirely.