In Drew's case, as I recall, he gained about $7 million, at a time when there was no guarantee he would ever make it to the bigs. All it cost him was a year in indy ball, instead of in someone's farm system. See also: Luke Hochevar.
Could other advisors/agents get these deals? Maybe, but if you were one of the guys with that capability, would you take the chance on someone unproven? And if you were Boras, having earned the rep to recruit the guys capable of commanding top dollar, why would you go looking for the lower tier guys?
Boras would probably tell you, the players create their value. He simply tries to capture if for them, instead of letting management keep it. He's not in a position to take a 10th round guy and get him millions. But not every top-10 pick gets what Alvarez was offered (and apparently declined). So you have to wonder, why is it that Boras can get these offers, while others just sign their clients up for slot money and then take 5% of what they might've gotten on their own?
Just remember, most of the grumbling you hear about Boras comes from ownership and the Commissioner's office (which represents ownership). The rest comes from people who think it's ridiculous that A-Rod makes $27 m/year.
Though why people find it absurd that A-Rod makes half what Brittney Spears makes, I don't know. I guess they'd rather see the Steinbrenners, whose $10 m original investment is now worth billions, do even better, and at A-Rod's expense?
Meaning, there's an unfathomable money stream running through MLB right now, and everyone is trying to enlarge their share of the pie. Both sides are fighting over it. For a disinterested observer to attack Boras to me indicates lack of understanding of what's going on.
I completely understand that the Commissioner's office is trying to cut Boras off at the knees, as part of its pro-owner role. I think what they're doing is wrong, but I understand why they're trying it. Why anyone would think this is fair play, given the impact on someone like Hosmer, I cannot understand.
Let's face it: There's not one of us out there who, if we had the chance to make the A-Rod deal for ourselves or our sons, wouldn't do it. (Go ahead, tell me how you'd take $10m instead of $27m "for the good of the game.")
For a lot of players, their shelf life is short and they may never get another chance to cash in beyond the one immediately in front of them. This is even more so for draftees, who have no guarantee of ever playing a single game in MLB, much less accruing enough time to become free agents.