This would appear to purely be a rental for the Royals, as Cueto is going to be a free agent after the season. Also, because he was traded, the Royals will not be able to offer Cueto a qualifying offer at the end of the season. FanGraphs projects Cueto to be worth about 1.5 - 1.6 WAR through the rest of the season, which could mean as much as two games difference in the standings. With Jason Vargas out for the year, Cueto will take his place. The Royals haven't had a true ace this year, so this move is definitely slated more towards having a strong Game One starter in the playoffs.
In return, the Reds get some very interesting talent. Finnegan has already seen the majors, but has been struggling with his control this year. Lamb is a 25-year old starter who's had a very strong year. Both Lamb and Finnegan are going to AAA for now, but will probably see the Major League roster in September.. Reed looks to be a talented arm, too, drafted by the Royals in the 2nd round of the 2013 draft. He'll report to AA, hoping to build off of this year's success. Finnegan may top out as a high-leverage reliever, but Lamb could very likely be a part of the Reds' rotation next year. Reed may be a year away or so from helping, but when he does he could be at least a valuable swing-man between the rotation and bullpen.
Did the Royals overpay somewhat? Perhaps not, especially as all three arms they sent away could end up being little more than bullpen arms or back-end starters. But the Reds can use all the talent that they can get right now. This is probably the best return they could have hoped for in exchange for two - three months of a starter that won't help them anymore this season. Cueto is easily going to be the Game One starter in the Division Series, and hopefully for the Royals, beyond. Therefore, he's really worth a lot more than two wins to them. If the Royals happen to win the World Series, this will be a pretty awesome move. Either way, the Reds have some solid arms for next season and beyond.