Take out the platters and smack 'em a few times with a hammer or gouge up the surface with a screwdriver, and that will reduce the odds of recovery to allowably small. It will cost around $2-5k to recover the data if you just throw it away without damaging it.
I should amend that to say
"It will cost around $2-5k to attempt to recover the data if you just throw it away without damaging it." But as stated previously, odds are better than 50/50 that a consumer-grade recovery place will be able to pull it off.
