Thats not how bonus buys work. They are simply equivilant to having spun the same number of spins at the cost of the buy divided by the stake. For example, if the stake is 20p and the bonus buy costs £20 then it will pay out what it would have paid out for those 100 spins, if it was going to pay nothing for those 100 spins then the bonus pays nothing, if it was going to land a big win or a high paying bonus during those 100 spins then thats what the bonus will pay out. Quite simple really..