As of this moment, the return on the trade is not yet known, but we'll have that information once it's available. The Cardinals are a contender in the National League Central, with its eyes on a third World Series win in the last decade, so you can expect whatever is coming back to Cleveland in this deal to be about futures seasons, not the next two months. St. Louis has a strong farm system, with a lot of good pitching prospects, so the Indians could be getting exactly what their own farm system needs.
Masterson has been with the Indians since being the centerpiece of the trade that sent Victor Martinez out of town in 2009. During that time he's appeared in 164 games for the Tribe, with 797 strikeouts and an ERA of 3.76. He was an All-Star in 2013. He's been dealing with a knee injury for most of this season though, and has an ugly 5.51 ERA in 98 innings, with one of the worst walk rates in baseball.
More on this trade, what it means for the Indians, and Masterson's contributions to the team to come.