At the end of the day a player's value is determined by what they ACTUALLY DO. And Detwiler has been a stud.
no doubt about it, but building a team is not about what they did, it is about what they WILL DO. You make a mistake paying for past good performance if it is not likely they will repeat it. With Detwiler, I think it is likely he will be a mid 3 ERA pitcher, based in large part on his peripherals. I have more confidence that, as long as he has an improving K and BB rate and gets a ton of groundballs, he'll be a very good pitcher. That's exactly what his 3.5 FIP says.
But you have to be one of the elite pitchers in the game, better than most hall of famers or likely hall of famers, to expect Detwiler will repeat a low 3 era the rest of his career. Among Glavine, Smoltz, Maddux, Schilling, Johnson, Clemens, Brown, Mussina, only Clemens and Maddux came close to that. Det has 30 starts or so since his call up last year. for now, I'd predict Det's ERA more off of defense independent measures than ERA unless you can come up with an explanation for why he beats his FIP or he shows a pattern over several full seasons.