#1) Koji Uehara
The Contract: $4.25 million for the 2013 season with a $5 million vesting option for 2014 that took effect upon his 55th relief appearance of 2013. (Vesting option was reported by ESPN long after the initial contract was announced, salary data via baseball reference.)
Alright, we all knew who would top this list before we even started. The rest of the league ignored the fact that the Japanese hurler had a 1.75 ERA and 14.33 K/BB ratio in 2012, and instead focused on his injury history and age (2013 was his age-38 season).
Those 29 teams made a humongous mistake. For the poultry sum of $4.25 million, Koji delivered a historically great performance. His K/BB (11.22) ratio in 2013 is among the top 10 for any reliever in a single season (minimum 60 innings) in the closer era and his WHIP (.57) is the greatest mark for any reliever with at least 60 innings ever. Tack on 13.2 immaculate play-off innings including 7 saves en route to a championship run and you have a player that will forever live in Red Sox lore. The fact that he was an all-star the next year was just gravy on top.
Koji is the gold standard for relief pitcher bargains and if they found a pitcher who could produce a third of what Koji did in 2013 the Red Sox would be ecstatic.