Long-term, big-money deals to relievers are the easiest way to get burned as a GM.
Justin Speier: 4 year, $18 million deal with the Angels.
Results? 2.88 ERA the first year, ERA over 5.00 the next two years, then released, a sunk cost.
I strongly disagree that a reliever can ever be a "proven commodity" their performance is so volatile that you can never really gauge what you're going to get from a reliever in a specific amount of time (sans Mariano Rivera, of course).
The funny thing is when people pine for the FO to waste copious amount of money on relief talent, those are the same people who will be the first to point the finger, bitch and moan about how "stupid the investment was in the first place".
Get a couple of the best type-B, mid-priced relievers available that suit the current Sox needs, and never, ever overpay for relief talent.