Bradford was 27th in the National League in 2006 in Relievers Expected Wins Added (WXRL) at 1.834. Trevor Hoffman led the league at 5.995, just a shade ahead of Billy Wagner at 5.948. The following free agent relievers posted higher marks last year:
Joe Borowski, 2.822 (#14)
David Weathers, 2.566 (#17)
Borowski and Weathers both closed for some or all of 2006 so that would account for some of the difference in WXRL relative to Bradford. Other free agent relievers include Scott Schoenweis, Ron Villone, Tanyon Sturtze, Keith Foulke, Eric Gagne, and others. Foulke is particularly interesting because he really grew to prominance with the A's under the watchful jacket of Rick Peterson.
Relief pitching is an extremely volatile market. Unless you're talking about the couple of dozen best pitchers in baseball, most of these guys are wildly inconsistent from year to year. Bradford has a history of back problems and ineffectiveness against lefties. He avoided recurrences of both last year, but I wouldn't bet $16 million that he's going to do it for the next three years as well.