Bonus buy RTP: what a feature buy really costs

Updated 5 Nov 2026. Examples use live figures from the Engine fairness API.

A bonus buy lets you pay a fixed price to start a slot's feature round directly, instead of waiting for it to trigger in the base game. On Stake Engine slots, each buy is its own mode with its own outcome table and its own RTP. This guide explains how to read that RTP, how to work out what a buy costs you on average, and how buys differ from boosted spins.

What a bonus buy is in numbers

Every mode on Engine has a cost, expressed as a multiple of the base bet. A base spin costs 1×. A bonus buy might cost 100×, 200× or more. When you pick a buy at a $1 base bet with a cost of 100×, you stake $100 for that single round.

Each buy also has an RTP, calculated the same way as for a base spin: the average payout of the mode divided by its cost. If that 100× buy pays an average of 96× the base bet, its RTP is 96%.

Expected cost of a buy

The single most useful figure for a buy is its expected cost: how much you lose per round on average. It is:

Expected cost = price × (1 − RTP)

At a $1 base bet, a 500× buy at 96% RTP costs you about $20 per purchase on average. Compare that with a base spin at the same RTP, which costs about 4 cents per round. The house edge per dollar wagered is identical; the amount wagered per round is five hundred times larger.

A live example: on Waylanders Forge by Valkyrie, the Powerspin mode costs 75× the base bet and returns 98.00%. Its expected cost is 1.50 base bets per purchase. A base spin on the same game has an expected cost of 0.020 base bets.

Why buys don't change the house edge, but do raise the stakes

It is common to hear that buying the bonus is "better value" or "worse value" than spinning for it. Measured per dollar wagered, the difference is usually small: buy RTPs on Engine tend to sit close to the base-mode RTP, sometimes a little above, sometimes a little below. What changes dramatically is the size of each round.

That has two effects:

• Your expected cost per round rises in proportion to the price. At the same RTP, one 200× buy costs the same on average as two hundred base spins.
• The swings get bigger. Buys go straight to the feature, where the large payouts sit, so a buy mode is almost always more volatile than the base mode of the same game. Many buys return far less than their price, and a few return many times it.

So the right way to compare a buy with a base spin is per amount wagered (RTP) and per round (expected cost), not one or the other.

How often a buy pays back its price

RTP is an average across all outcomes. A more intuitive figure is how often a single buy returns at least what it cost. We call this the pays-back-its-price rate and show it as "1 in N".

For the Powerspin mode on Waylanders Forge, a purchase returns at least its price 1 in 16 buys. The remaining purchases return less than they cost, and the average across all of them comes to the 98.00% RTP.

This rate is a good reality check. If a buy pays back its price only once in five tries, then four out of five purchases return less than you paid, and the occasional large result is what lifts the average towards the RTP.

Bonus buys versus boosted spins

Not every paid mode is a bonus buy. Many Engine slots also offer modes that cost a little more than a base spin and change the odds of the round in some way, for example by raising the chance of triggering the feature. We classify modes by cost:

The 20× line is our own threshold, applied consistently so that modes from different studios can be compared. Studios use many names for these modes; the cost is the reliable signal.

Boosted spins work the same way mathematically. They have their own RTP and their own expected cost, which is the price times one minus the RTP. A 2× boosted spin at 96% costs 0.08 base bets per round on average, double the base spin at the same RTP.

How to compare buy modes

1. Start with RTP. Among buys you are considering, a higher RTP means a lower average cost per amount wagered.
2. Check the expected cost per round. Multiply the price by one minus the RTP. This is what each purchase costs on average at your base bet.
3. Look at the pays-back rate. It tells you how often a single purchase comes out at or above its price.
4. Look at the max win and its odds. On some games the top prize is only realistically reachable from a buy. The max win odds guide explains how to read those figures.

The bonus buy calculator does all four for any mode and base bet you choose, using the published outcome tables. The bonus buy slots list ranks every game with a buy mode so you can compare them side by side.

How buy RTP is calculated

Each buy mode on Engine has a published outcome table: a list of possible results, each with a payout and a weight. The average payout is the sum of payout times weight divided by the total weight, then divide by the mode's cost. That gives the RTP on the same footing as a base spin. The game page also shows the catalogue's published figure, so you can compare the two. The RTP guide walks through the calculation step by step.

Frequently asked questions

Is a buy with a higher RTP than the base game better value?

Per dollar wagered, yes, slightly. Per round, it still costs much more on average, because you are wagering the full price in one go.

Does buying the bonus give the same result as triggering it naturally?

Not necessarily. A buy mode has its own table, which may differ from the feature you would reach from the base game. Compare the mode's own figures.

Are buys available everywhere?

Availability depends on the casino and local rules. This site only reports the math of each mode as published.