Attack Boost is an activated skill, like Knockback or Double Attack. The unit that has the skill Attack Boost (Gender/Series) has to attack, and has to pass the % chance to activate it. It then boosts all relevant units. It's not like Iron Wall, which activates on any enemy turn, and it's not constantly active like it normally is in a quest.
I think the sudden rise in price for those units is this event and their incredible usefulness in boss slaying.