Right now, Roy Halladay is a necessity in New York: Life might go on next year without him, but things would be very easy with him. That said, if the Yankees want to get him, what would they need to give?
Multiple sources have claimed that "for the right trade," Roy would waive his no-trade clause. Here is what the "right trade" might look like:
The Jays have expressed interest in the Yankees "MLB-ready" pitchers, like Phil Hughes, Joba Chaimberlain, and Ian Kennedy. They would also like a prospect suck as Austin Jackson, or Jesus Montero. Melky Cabrera may have to migrate north of the border, too, as the Jays have looked at him to plug the hole they have in right field.
What do the Yankees want to give the Jays?
Cross off Jackson from the list, the Yanks will not under any circumstances trade him.
I would hate to see Montero go, as he looks to be Posada's replacement. However, the Yankees do have a few, less promising catcher prospects in the farm.
With Joba, Hughes, and Kennedy's futures in air, they will most likely go. If not all three, at least two of them would go.
Melky leaving might open a hole in center for the Yanks. However, Austin Jackson could be a candidate to take over that role, and Brett Gardner is always an option.
Here's the trade I could see working:
Joba Chaimberlain, Phil Hughes, Melky Cabrera, Austin Romine, and Cody Ransom for Roy Halladay and Brad Emaus. Then, Austin Jackson and Brett Gardner could platoon in CF until Jackson seems ready. The Yankees could sign a pair of relievers to fill Joba's and Hughe's jobs, and then bring back Pettite and sign another started to fill the rotation.