Ah, this is a tricky question, because there’s no right answer. It may be that you shouldn’t opt for a bonus buy at all if you’re playing on a limited budget.
True you could go for the least costly Bonus Boost option, but that “only” doubles your chances of triggering free spins; i.e. it doesn’t guarantee anything. But the guaranteed option costs 100X your bet, so you’d have to decide if a sure thing’s worth it to you.
And since there’s certainly unique fun in facing off against Billy the Bully, and the two Bonus Buy Battle modes are your only options to do so, you may want to try at least one of those at a low bet rate, to see what all the rage is about. Either of these, actually is what we’d most recommend for their originality factor, though they’re not a must, and again, only if you can afford it.