Ok. I think you cannot compare the Bailey, Melancon and Kimbrel trades in term of packages given up.
Bailey trade: Reddick was, according to BA, only once rated in the top 100 prospect list (2009: 75th). He was of course rated 3rd in a Red Sox system that then was not close to the quality of today. Neither Miles Head nor Raul Alcantara ever were rated in their respective careers in the top 100 list. Were they close to it? I cant remember. Boston also got Ryan Sweeney who was ok as a part-time player.
Melancon trade: Jed Lowrie was a semi-established MLB player (2.5 WAR over 4 seasons) and Kyle Weiland never cracked the Red Sox top 15 prospect list and therefore never appeared on a top 100 list.
The package given up for Craig Kimbrel is much more impressive (two top 100 prospects) plus one that could crack the list in 2016.
Plus we can all agree that the Melancon and Bailey trades were bad trades from a Boston perspective.
I don't think either the Melancon or Bailey trades were bad from a Boston perspective. Trades are made to address a particular team's weakness at a specific point in time. You ideally strive to never give anything away that will come back to haunt you, but you have to address weaknesses to compete. Ultimately, you're always exchanging sets of probable outcomes, as they're known at the time the trade is made.
For example, in 2011, the Sox just missed the playoffs. They had Gonzalez, Pedroia, Youkilis, Ortiz, Ells, Crawford, Papelbon, Bard, Beckett and Lester in their primes, more or less. They were hurt by poor SP depth down the stretch. And perhaps chicken and beer.
In 2012, there was going to be roster turn over. (Varitek/Wakefield/Papelbon et. al. were leaving and Lackey was getting surgery.) The 2012 plan was to augment the SP by converting Bard, bringing back a healthy Buchholz, and taking a flyer for the 5th spot. They could have ended up with 3 or 4 excellent starters, plus their great offense. But converting Bard, having Jenks injured, and not resigning Papelbon weakened the bullpen. So they had to trade to address this.
Reddick was a 24 year old good defensive RF, who was a bit above league average in offensive production, but mostly due to power, not OBP. They gave up two prospects. So that's some upside and long term control of it going to the A's.
The Sox got back a 26 year old Sweeney, who was good defensively, and did everything well but hit for power. Sort of the anti-Reddick. (They also signed Cody Ross.) So that's kind of a wash. They additionally got Bailey, who was good when not injured and seemed healthy. They also traded for Melancon resulting in a Bailey/Melancon/Aceves bullpen with Bard/Tazawa/Miller, et. al. in the wings. (Aceves seemed very legit in 2011.)
I won't break down the Melancon trade, but that's what those trades were - moves made in context dependent on how complete the team was and how competitive they were likely to be.
They were ultimately Valentined (and Acevesed and Barded), but the point is those trades weren't made in some sort of abstract "I'll trade you my number 9 for your number 8" way.