The Adam LaRoche trade when looked from the Red Sox point of view was the opportunity to pick up a player who can A: help the Red Sox against right-handed pitching and B: give them additional depth to rest Mike Lowell. The cost was basically nothing from the Red Sox point of view. They gave up two prospects who never were going to sniff the grass at Fenway as players and the $3 million remaining on LaRoche's 2009 contract is pocket change to Boston. In fact - with LaRoche a free agent at the end of the year - the draft pick they get if he signs elsewhere may yield a much better prospect than either the Red Sox gave up (combined).
From the Pirates point of view - this was a salary dump. That $3 million to them is a very big number.