Every off-season a dozen middle relievers will be paid quite handsomely for their services. Why? Because teams seem to forget that most middle relievers are either failed starters or closers. That and the need for immediate bullpen help outweighs the idea of taking risks on more cost efficient options. With that in mind here are a few relievers likely undervalued by their current clubs that could be had for less than that four million per season you may spend on someone else.

Chris Sampson (ranks 7th in tRA for relievers)
No, I did not realize Sampson was quite this good either. A 3.67 FIP and sub-2.3 tRA is quite impressive for the 30 year old. Sampson does not strike too many out (4.65 per nine) but he doesn't walk many either (1.76) and his strand rate should be better than 63%. As for his batted ball tendencies: 14.3 LD%, 56.9 GB%, and 7.5 HR/FB%. Why would he be undervalued? A 4.25 ERA is a nice starting point. Also don't forget that Sampson failed as a starter earlier this season, perhaps Ed Wade remembers as well.

Darren O'Day (ranks 28th in tRA for relievers)
The side-arming former Florida Gator is a lot like Sampson. O'Day gets groundballs (54.9%) but with higher ratios of strikeouts (6.02) and walks (2.91). O'Day's 4.57 ERA is nasty, just nasty, and that .335 BABIP despite a 16.7 LD5 is awful. O'Day is clearly too hittable to become a reliever in a good bullpen. Hopefully Tony Reagins realizes so and deals him to your team.

Neal Cotts (ranks 127th in tRA for relievers)
Cotts is on the bottom half of tRA rankings, however he gets just about as many strikes swinging as Jonathan Papelbon and Joba Chamberlain. His strikeout and walk ratios are much improved and the only notable reason is an increase in velocity. I don't have a reason for the bursts, but they are apparently legit; from 90 to 91 on his fastball, from 82.5 to 85.5 on his slider, and from 83 to 84 on his change. Cotts issue is the homerun, as it has been (along with the walk) for the majority of his career. Of course, Cotts has also pitched in two ballparks prone to the longball, put him in Detroit or Cleveland and the story might change.

Kevin Gregg (ranks 81st in tRA for relievers)
Despite the nudge in walks and decrease in strikeouts Gregg has found a way to lower his homerun rates and maintains a solid FIP. He's not a bullpen ace, which is ironic because the Marlins knew as much when they made him into a closer, but he's not a bad pitcher either. Teams can do far, far worse in set-up roles than Gregg.

Trevor Hoffman (ranks 95th in tRA for relievers)
It seems amazing the all-time saves leader can be undervalued, but that's exactly what I would peg Hoffman. His ERA and FIP will be the highest in yearsbut Hoffman has quietly recorded his best K/BB ratio since 2004 and broke the 1.0 K/IP ratio for the first time since 2003 while walking less than 1.8 per game for the first time since 2004. The problem for Hoffman has been homeruns, 1.70 per game. When Hoffman has pitched post-June he's racked up 15.1 innings, allowed two walks, two homeruns, and struck 14 out. I am not sure if that's a sign of things to come, but I would expect Hoffman to at least match his 4.01 FIP, just perhaps not in San Diego.

References:

Statcorner and their outstanding leaderboards.

FanGraphs and their outstanding ratio stats.

B-Ref and their outstanding gamelogs.