Earlier today I wrote why Matt Garza's trade value may be higher than I think, or rather higher than the trade value numbers show. Since I wrote about why they may be higher it only makes sense to also spell out why they may be lower. I'm going to do the same thing for Ryan Dempster tomorrow, but some of that is written here already (why he's worth more).
While Matt Garza threw 133 innings for the Twins in 2006 and 2007, he didn't have his first full season until he was traded to the Rays in 2008. Since then, his FIPs have been 4.14, 4.17, 4.42, 2.95, 4.16. These are a lot alike with the exception of his first season in Chicago.
It's certainly more valuable than his best season was more recent, but one might look at these numbers and be tempted to throw out 2011. I would never encourage anyone to do this. The numbers happened and they matter. They tell us something, but it's easy to see how some team may look at the years 2008 through 2012 and see that he's been about a 4.2 FIP pitcher with the exception of one season (2011 when he was damn good).
Fangraphs bases its WAR on FIP and his fWAR has been (since 2008) 2.9, 3.1, 1.6, 5.0, 1.2. He's essentially on pace for about a 2.5 fWAR season if he reaches his ZiPS projection the rest of the way. Not surprising since it's based on FIP, these numbers are all relatively similar with the exception of one season.
He has a 108 career ERA+. That's only 8% better than the league. His ERA+ since 2008 has been 119, 110, 100, 116, 99. Since 2008 his ERA+ is 109. For comparison, Ryan Dempster's is 118. Dempster has had 2 full seasons (2008 and 2009) in which his ERA+ was higher than Garza's best season. Dempster's this year is likely to be higher than any of Garza's best.
Garza has 13.8 fWAR since 2008. Dempster has 18.4.
Baseball Reference uses runs allowed to calculate WAR. They factor in fielding by using the team defense and then assigning that to each pitcher based on his innings pitched. Garza's rWAR since 2008 has been 3.1, 3.3, 1.3, 2.5, and .5 this year. When you get away from the fielding independent stats, his WAR last season wasn't even the highest in his career. Looking at it this way makes two seasons stand out (2010 and 2012). The others have been close to the same while those two years have been quite a bit worse.
He has 10.4 rWAR since 2008. Dempster has 15.8.
Since 2008 Baseball Prospectus has Matt Garza being worth 7.6 WARP while Dempster has been worth 13.1 WARP.
Ryan Dempster has been a better pitcher over the last 5 years. It's not really even that close and it doesn't matter which value stat you look at. Dempster has only a few months left on his contract and someone wouldn't be crazy to expect acquiring Dempster to be a better trade than one for Garza. Dempster could probably be had for a little less, but maybe not.
If people believe Matt Garza is worth a whole lot, why do people think Ryan Dempster is going to be worth a lot less? Just because Garza has one year at approximately $12 million in salary left? I don't know. Garza isn't as good as a lot of people think and there's definitely reason to believe he may be worth less in a trade than the numbers show.
I wouldn't bet on that. I also wouldn't bet on him being worth more.