1) No Type-A Relievers
These players will cost the Rays their first round pick as well as the financial terms necessary to hook the arm. That means Billy Wagner, Darren Oliver, Octavio Dotel, Jose Valverde, Mike Gonzalez, Rafael Soriano, LaTroy Hawkins, Rafael Betancourt, Kevin Gregg, and John Grabow are all no-goes. Yes, the Rays have two first round picks, however, the compensatory pick is more than a dozen picks away from the 2010 first round pick, meaning if the Rays want a Type-A player it will come at the cost of a mid-first rounder, not a late-first rounder.
Now, there is a potential exception. If the team of a select player chooses against tendering an offer of arbitration which waives the compensatory pick. That being stated, if the Rays were to sign a player before the arbitration acceptance deadline (which falls within the first week of December) then they would give up the first round pick no matter the other team's intentions.
Some of those relievers are good players, but giving up the sixteenth pick (technically; virtually the seventeenth pick) for 60 innings is worthless. That slot has produced some fantastic ballplayers. Over the last four years: Bobby Borchering, Brett Lawrie, Kevin Ahrens, and substance abuser Jeremy Jeffress. Some other notables: Chris Volstad, David Purcey, Nick Swisher, Jason Jennings, Kip Wells, Lance Berkman, and Shawn Green. A few true stinkers too, but for the most part some fine talent.
2) Injuries Are Sometimes a Necessary Evil
Especially with pitchers. If guys like Kiko Calero were 100% dependable then they would be outside of our price range. Thankfully they aren't in either case. This is where luck and medical staffs come in handy. If you sign three relievers with about 60% chances of pitching more than 45 innings, then the chances of all three doing such is 21% and the chances of none of them doing it is 6.4%.
J.J. Putz, for instance, has bone spurs, tendon fraying, and gingeritis which lead to downed velocity and ineffectiveness but could make for an interesting non-roster invitee. Same can be said about Chad Cordero who quickly disappeared after a few years as a rosterbation champion.
3) Trading Prospects for Relievers
Not a good idea under most circumstances. Trading Reid Brignac for a young catcher with a few years of control left and having a young relief arm thrown in as an equalizer is one thing, trading a young prospect for a reliever straight-up is generally not smart. For instance: I absolutely detested the Orioles/Dodgers mid-season swap from the Dodgers perspective. They gave up third baseman Joshua Bell for George Sherrill. The latter was fantastic for the Dodgers - providing 0.4 WAR during half a season - meanwhile the former posted a .397 wOBA in Double-A and possesses the nickname "Baby Kemp". You don't have to guess which side is probably winning that deal.
Summary
Of the top 15 relievers this year, only three were free agent signings and those three were Kiko Calero, Chan Ho Park, and Trevor Hoffman. Seven were dealt for, four were drafted (three as starters), and one via the Rule 5 draft. Relievers are often overvalued because of sentimental value - i.e. when a team loses in the late innings - but the truth is even the best closers are only worth two-to-three wins in any given year.