There are 2 kinds of Boost Bonus, the first one is Affinity Bonus, when u use a same elemental monster to boost, u get bonus 50% experience more, the second one is a randomly appearance Boost Bonus , sometimes when u boost your monsters, u will get this bonus experience.
Boost Bonus = +23%
Big Boost Bonus = +50%
Super Big Boost Bonus = +100%
Extra Bonus = ??
note: Boost 5 monsters will give u a higher chance to get the second-type boost bonus