I really like the trade.Others have stated why didn't the twins just get Ryu?The difference is if Maeda doesn't pan out with the Twins, the twins will not be on the hook for an additional 17 million per a year.
Maeda will get enough of a leash, especially the first year or two, that he's virtually guaranteed some of those incentives, so the cash difference is less than that. And of course, we lose Graterol's current value, which essentially adds ~$20+ mil to Maeda's overall cost even if Graterol doesn't pan out. (Ryu didn't even cost a draft pick.)
There's also the upside element. Ryu just completed a 4.8/5.1 fWAR/bWAR season, while Maeda's best so far is 2.9. Maeda's definitely cheaper, but it's because he seems to come with less upside.
Edit to add: not that I'm arguing for Ryu over Maeda, but the cost difference between them seems fair. We essentially used a prospect to save money, and further perhaps traded some upside for less financial risk.