Well, we can look at what the Red Sox have been willing to give up in trades for closers:
Gagne: Murphy, Gabbard, and Beltre
Bailey: Reddick, Head, and Alcantara
Melancon: Lowrie, Weiland
Hanrahan: Melancon, Sands, Pimental, DeJesus
If you take a central tendency across those trades, it looks like they're willing to offer a good, young "10th player" with some upside (Murphy, Reddick, Lowrie), plus a top minors pitching prospect who projects as a swingman-type or set-up guy (Gabbard, Weiland, Melancon), plus a low-minors toolsy type (Beltre, Alcantara, Pimental).
And, those packages were for players who were only under control for a year or less (Gagne, Hanrahan), had significant injury concerns (Gagne, Bailey), or very shallow track records (Melancon). So, by extension, they should be willing to give up significantly more than, say, Murphy, Gabbard, Beltre, for 4 years of perfectly healthy, in his prime, proven, and dominant Craig Kimbrell (or 2 years of Greg Holland or Wade Davis who you'd want to kick the tires on before giving Atlanta their asking price).
So, what's a better deal than Murphy, Gabbard, Beltre? JBJ, Wright, and Rijo? Probably not enough. But, does substituting Owens for Wright get you there? Is that too much? Is it out of line with what they paid for Gagne or Bailey, considering the difference in quality and control between those two and Kimbrell?