Although Cueto may have come a little cheaper than he otherwise would have considering that he is a free agent this offseason, he still came at a good cost for the Royals. The three prospects in question, Brandon Finnegan, Cody Reed and John Lamb, all of them left-handers, all have a chance to be major league starters (although Finnegan could also project highly in the bullpen).
But this trade is worth it for the Royals. Cueto helps quickly improve a rotation that has struggled thus far amid the team's success, with Yordano Ventura being a major disappointment thus far while Edinson Volquez and Chris Young have struggled to pitch deep into games, taxing a strong bullpen. Cueto may not be pitching at the level that he did a year en route to being the Cy Young runner-up, he still has been pitching very well and certainly better than anyone in the KC rotation. He also is an innings-eater which will be huge for the bullpen down the stretch.
Given that James Shields was priced out of the team's range last offseason, Cueto is all but certain to be too expensive for the Royals to bring back next season. However, as Jeff Passan notes, with Alex Gordon being a free agent this offseason and a number of key contributors hitting free agency after next season (Eric Hosmer, Mike Moustakas, Alcides Escobar, Lorenzo Cain, Danny Duffy and Greg Holland), the window is not likely to be more open for the Royals to win their first World Series since 1985.
With the reputation that the franchise has built under owner David Glass and under general manager Dayton Moore, it generally would be a stunner for the team to pull off a move like this one. It would have been the easy way out for them to simply sit on what they have and hope for the best, and they could have won the American League by standing pat. But now with a horse like Cueto in their stables, they have to be the clear-cut favorite to return to the Fall Classic.