There's a reason Headley in 2nd among all 3B in WAR this year, and doesn't have anything to do with his height. Nobody here is taking into account how much San Diego's home ballpark suppresses offensive stats.
Before even looking at park factors though, Headley beats Callaspo easily in wOBA for their careers (.331 to .319) as well as this season (.352 to .319). Looking at the park factor, the difference becomes larger, shown in his numbers on the road (.279/.382/.481 this year, .300/.366/445 career), or more accurately in his wRC+ of 129, which puts him above Curtis Granderson, just behind Prince Fielder, and above every Angels hitter except for Trout and Trumbo. In a better home park I have little doubt he could be a 20/20 player and possibly push for a .400 OBP. Headley vs Callaspo should not even be a debate.
Being under control through 2014 and in his prime years, he's absolutely worth multiple top prospects. If the Padres undervalue him as so many others do (which I doubt), whoever ends up acquiring him could get a steal.