Prospect trades don't always work out that well.
Going off my Expo memories, Pedro Marinez, was traded in the 1997 offseason after winning the Cy Young, with a 1.90 ERA, 13 Complete Games, and 305 Strikeouts. He was going to be a free agent after 1998.
Halladay might have similar value, since he is around for two years. He was not as good as Pedro, who from 1997-2003 was one of the greatest pitchers of all time.
The Expos traded Pedro for Carl Pavano and Tony Armas. Carl Pavano had awful mechanics whipping his arm against his body, and was an injury waiting to happen... he had a few good moments which he parlayed into fooling the Expos and then the Yankees.
Tony Armas was a talent, good stuff, but the typical million dollar arm with a 10 cent brain... he then got injured, and lost a bit, and his career went nowhere.. the Pirates thought it would be smart to give 3M a year for a couple of years.. and he did nothing.
Personally, I think there is so much unpredicatability with young arms and how they develop, that you trade Halladay for position prospects as the key of the deal. They are much more predicatable.