If you wait until the 7th round, you won't get him. If he is picked in the 10th round, it was probably an auto-draft where most didn't rank their players.
He will trail only Randy, Curt and Pedro in K's. Those SP's are first rounders. He may not get the as many wins as those guys, but he will make a big enough impact to be an elite fantasy pitcher. If you draft Randy/Curt/Pedro you probably don't need Prior in the late 3rd or early 4th. If you took a couple of bats in the first 2 rouinds and Barry and Roy are gone, Prior is the guy I would target in the late 3rd early 4th round. You want a guy that can stand above the competition as your staff ace. Prior is the best option after the top 5 SP's. I think he is a good value around pick 34, exactly where I grabbed him.
* Steroid induced record