In trying to guage what kind of return we can get for Will Ohman, we can look to the recently completed trade between the Marlins and Mariners. Seattle sent 38-year old reliever Arthur Rhodes to the Marlins for 22-year old pitcher Gaby Hernandez. The young Hernandez was rated as the 6th-best prospect in the Marlins organization this year and is considered a mid to back end of the rotation starter who is almost major league ready.
Rhodes is having a good year after coming back from Tommy John surgery last season, but I would consider Ohman a more valuable bullpen arm that him. We should be able to expect a higher return than a major league ready back-end of the rotation starter (though, we could probably use a solid innings eater like that somewhere in our rotation instead of the four and out back-end guys we have now).
The rumor yesterday had us swapping Ohman for the Red Sox' Daniel Bard, a former first-rounder and a guy who is more suited to the bullpen than the rotation. Perhaps the Braves are holding out to see what these other lesser relievers bring back before they pull the trigger on an Ohman deal.