Well, buying 30% of the supply is probably not possible from casually going on exchanges.  You'd have to go to some extreme lengths paying crazily high prices for peer-to-peer transfers with people who are reluctant to sell, and then the price would absolutely rocket.

After destroying the private key associated with it, you'd have decreased the supply by 30% (which you would have basically done by holding it anyway), which should keep the price extremely high.

That said, if people were worried about things like that happening again, FUD could cause the price to drop a small amount of the way back down.