Hunter Pence is under team control through the 2013 season as an arbitration-eligible player. He makes a pro-rated portion of $6.9 million for the rest of this year and can expect around $25 million in 2012 and 2013, combined, via arbitration. The Phillies can expect Pence to provide them with around 8 wins above replacement (fangraphs' replacement level) over the life of his contract. At $5 million/win, that's $40 million. Subtract the $27 million Pence will make, and they have a +$13 million asset.
According to +Keith Law's mid-season prospect rankings and the prospect surplus value series at BtB (
http://www.beyondtheboxscore.com/2010/7/13/1567771/prospect-surplus-value ), the Phillies gave up $55 million worth of prospects.
So, unless the Astros are kicking in ~$40 million; the Phillies got fleeced big time.
EDIT: if you use BA's mid-season list, the value drops to $45 million --
http://www.baseballamerica.com/blog/prospects/2011/07/midseason-top-50-prospects-list/