Skunk nailed this one.
You want to get into the nitty-gritty of valuing pitchers, you have to know how many pitches you're buying. Just as a random example, the '09 Christy Mathewson is credited with 14.6 pitches/inning... and the '74 Nolan Ryan gets 19.0. That extra 4.4 pitches per inning adds up to a whole lot.