Medina's still settling into the team but his performance already justifies his base compensation. (The abridged case: he's 69th in compensation, 71st in NPxG+xA.) You could argue about the DP slot and transfer fee, but we don't actually know the transfer fee and the DP slot would probably otherwise have gone to Berget this season, in which case using it on Medina saves us $300k cap space.
As for promise, that's part of what we're investing in. There are few if any better players Medina's age in MLS. Of guys who are on his level but younger, Barco cost at least three times as much and Davies will be an MLS record sale when he leaves. Slightly older guys who are on the level we'd like to see Medina reach in a year or two, like Almirón, Elis, and Villalba, all showed huge improvement between their first and second seasons. Give Medina a year to develop under Torrent and then tell me how you feel about the signing.