This might seem an odd question, considering Zack Greinke is one of the best pitchers in baseball and was one of the hottest commodities on the market over these past few weeks. Of course he should bring back a greater haul than Matt Garza!
Upon closer inspection, though, I think the argument can be made that the Rays could do better than the Royals did. Greinke forced the Royals' hand, demanding a trade and lowering the team's leverage, and the Royals' general manager, Dayton Moore, isn't known for crafting brilliant transactions. Greinke brought back a haul of Alcides Escobar, Lorenzo Cain, Jake Odorizzi, and Jeremy Jeffress, a collection that seems to underwhelm most Royals fans.
When PGP first calculated Matt Garza's trade value at the beginning of this off-season, he pegged Garza as worth around $21M in return value - or in other words, one top 50 positional prospect. Last year, Alcides Escobar was ranked by many people as one of the top 50 positional prospects in baseball - with some placing him as high as 16 overall - but his value got hurt by poor performance in the majors this year. The other players in the deal also have value, especially Cain and Odorizzi, but I haven't calculated out exactly how much.
Garza has a lot going for him. While not as good as Greinke, he's still a darn good pitcher, is
slightly cheaper ($5M as opposed to $7.25M $13.5M, h/t BurGi), is under team control for one year longer than Greinke, and has an incredibly shrewd general manager pumping up his value. Garza is far and away the best available starter on the market right now (and conceivably the best starter that will be available for the next couple of years), and Friedman has already claimed he doesn't want to trade him; he'll have to be blown away to make a deal.