Here is a list of what Philadelphia has traded away for Cliff Lee, Halladay, Oswalt, and now Pence. I've attached a couple non-sabermetric notes to each name. With respect to some Philadelphia prospects list in 2009, the Phillies have traded away 7 of their top 10 prospects since that time.
What has been the return? In the majors - a young, currently #3 starter, a #4 starter, a demoted young starter, and a backup catcher; for the minors, it seems like a couple decent prospects (still) and a couple quad-A guys.
I'm not sure how attractive this return is for Lee/Halladay/Oswalt/Pence. Perhaps we have a tendency to overvalue prospects - but if anything, selling GMs may be forced to accept quantity over quality.
Player Level Age Notes
Jared Cosart A+ 21 Walks are up in A+; K/9 down ~30% from A; 1.30 WHIP
Jon Singleton A+ 19 .798 OPS in 316 ABs; 14 doubles/9 HRs; 56/83 BB/K ratio
Josh Zeid AA 24 5.65 ERA; HR/9 up 100% from A; SO/BB reduced 50% from A
J.A. Happ MLB 28 #4 starter in Houston; 6.12 ERA in 110 innings in 2011
Anthony Gose AA 20 47 steals as a CF in AA; .746 OPS; 13 doubles/10 HRs in ~350 ABs
Jonathon Villar AA/A+ 20 27 steals as a SS; .727 OPS; 17 doubles/11 HRs in ~400 ABs
Kyle Drabek AAA/M 23 Started season in Toronto; put up a 5.70 ERA; demoted and not faring well since June in PCL
Michael Taylor AAA 25 .858 OPS in 266 ABs in PCL; split time between LF/RF
Travis D'Arnaud AA 22 .923 OPS in 333 ABs, 26 doubles/13 HRs
Carlos Carrasco AAA 24 #3 starter in Cleveland; first full season; had an impeccable June/July but has since faltered, walks up.
Jason Knapp A 20 Out for the season; second surgery since trade; 14.9 K/9 in A-RK ball in 2010
Lou Marson MLB 25 Backup catcher in Cleveland; .637 OPS in 128 ABs in 2011