I do look at Garza as costing about the same as Anibal Sanchez did last year, namely a pacakge headlined by one top 50 prospect. If Sanchez is worth Jacob Turner, in no way do the Sox get Garza without giving up either Webster or Barnes.
Epstein was also very enamored with Webster last July, insisting on getting him for Ryan Dempster (which is why the dal with LA fell apart). Barring a drastic change in his opinion, I would assume Webster is probably a necessity if you want Garza. That makes it a tough call, but I think I pass.
The smarter move would be to go for an effective bullpen. The dumbest thing to do is emphasize closing experience over actual effectiveness. Just get someone who can get outs and let the ninth sort itself out, especailly since the sox have a couple arms (Uehara, Tazawa specifically) capable of getting 3 outs before they give up 3 runs. I'd rather see the Sox get someone like Aaron Crow or Luke Hochevar over someone like Kevin Gregg...
“Listen to the mustn'ts, child. Listen to the don'ts. Listen to the shouldn'ts, the impossibles, the won'ts. Listen to the never haves, then listen close to me. Anything can happen, child. Anything can be.”