You can not in any way shape or form compare events to intel projects. Intel projects from a single empire can destroy up to 12 planets in a single turn (though more likely 11 or fewer, as the attacker will need at least 1 CI), and then 12 planets in each subsequent turn, turn after turn. Events could never do that. I think there is a limit of 1 event per turn.
Also, more expensive means harder to defend against as well. So, those planet busters become even more of a problem if you make them really expensive.
