Roy Halladay is 32 years old. In about three years, he's going to not be as good. In about five years, he's going to be league average.
If we did somehow trade for Halladay (it'd cost us a bundle to convince the Jays to trade within the division), not only would it require us to give up some very, very valuable pieces (Joba Chamberlain, Phil Hughes, Jesus Montero our 40 HR power catcher in the minors, Austin Jackson our 5-tool AAA outfielder, etc.) but we'd have to sign Halladay to an extension once we got him. That extension would likely be for 5-6 years, and would be at 20 million dollars a year, 120 million in total. For a pitcher who's not going to be all that great in 5-6 years? I don't really think it's worth it.
And those of you worried that the Red Sox are going to get him...needn't worry, as the Red Sox hardly need starting pitching right now - and if the Red Sox wanted to get him, they'd probably have to deal their star 1B prospect Lars Anderson - and given the fact that Mike Lowell is on his way out, they'll need a 1B since Youkilis is probably going to play third.
Just let him go to some NL team like the Phillies where he'll never face us. Even if he did go to the Phillies and we faced the Phillies in the WS the rest of the Phillies' rotation sucks anyway so whatever