Discussing the Mariners and David Aardsma today, David Cameron wrote, "There is a school of thought that closers garner more in return when moved at the deadline than in the offseason, though I haven’t seen much in the way of real evidence to support the assertion."
Looking at the last three seasons and offseasons, let's look at examples with the help of MLBTR's Transaction Tracker. This post won't answer Cameron's question in a statistical sense, but it may help shed some light.
I found four closers who were traded during the 2008-10 seasons: Octavio Dotel, Matt Capps, George Sherrill, and Jon Rauch. I've omitted the trades of Brian Fuentes, Chad Qualls, Kerry Wood, Billy Wagner, and Joel Hanrahan, as those five either weren't closing at the time of their trades or had prohibitive salaries.
- Dodgers acquired Octavio Dotel and $500K from Pirates for James McDonald and Andrew Lambo. Though both players had fallen out of favor with the Dodgers, this viewed as a steal for the Pirates. Dotel once fetched Kyle Davies from the Braves in a different trade, also a fine return.
- Twins acquired Matt Capps and $500K from Nationals for Wilson Ramos and Joe Testa. The Twins acquired a season and a half of Capps, but with Ramos regarded as at least a regular in the near future, Minnesota had to overpay for Washington's closer.
- Dodgers acquired George Sherrill from Orioles for Josh Bell and Steve Johnson. The Dodgers had potentially acquired two-plus seasons of Sherrill, who had a 2.40 ERA when the deal was struck. But Bell was a well-regarded prospect and was anointed Baltimore's third baseman of the future at the time.
- Diamondbacks acquired Jon Rauch from Nationals for Emilio Bonifacio. Bonifacio was Arizona's sixth-best prospect, but I'd expected Jim Bowden to ask for more for two-plus seasons of Rauch. It's fair to say the D'Backs didn't have to overpay for Rauch, though his closer pedigree only involved six saves before that season.
Five closers were acquired during the last three offseasons, not including this one: Rafael Soriano, J.J. Putz, Kevin Gregg, Jose Valverde, and Brad Lidge. We're omitting Huston Street and Matt Lindstrom, who had lost their closer jobs before being dealt. Gregg was on the outs in Florida but we'll include him.
- Rays acquired Rafael Soriano from Braves for Jesse Chavez. This isn't a good example, as the Braves were backed into a corner by Soriano accepting arbitration after they'd already signed Billy Wagner and Takashi Saito.
- Mets acquired J.J. Putz from Mariners. This was a huge three-team deal. The Mets received other players, but didn't give up premium pieces to acquire Putz. Putz was coming off a rough '08 season though.
- Cubs acquired Kevin Gregg from Marlins for Jose Ceda. I think most viewed this as a win for the Marlins at the time.
- Astros acquired Jose Valverde from Diamondbacks for Chad Qualls, Juan Gutierrez, and Chris Burke. At the time, I questioned whether Josh Byrnes got the best possible return for two seasons of Valverde.
- Phillies acquired Brad Lidge and Eric Bruntlett from Astros for Michael Bourn, Geoff Geary, and Mike Costanzo. At the time Bourn/Costanzo was generally considered a high price paid by the Phillies.
More data points would be nice, but this is a start. Teams overpaid to acquire Dotel, Capps, and Sherrill midseason, though Ned Colletti authored two of those deals. The prices did seem lower for Putz and Valverde during the offseason, but not necessarily for Gregg or Lidge. I think the conventional wisdom has value - contenders are more desperate for relief help during the summer, and with no free agent alternatives they're willing to surrender slightly better prospects for closers.