Fair value, but why would Anaheim consider this deal? The Ducks are filled with old aging veterans that are going to be over the hill in a couple of seasons, like Perry and Kesler. Instead of trading players like Josh Manson, they should be looking to add to their young crop of players - Rakell, Lindholm and Manson etc.
A player like Jake Gardiner holds no value to an organization like Anaheim. He is about to be a Free Agent next summer, and Anaheim isn't going to win a cup in the next couple of seasons.
Plus,
Manson > Gardiner
2018 3rd round pick = Josh Leivo (a guy who has requested a trade, so might be available for cheap)
Dereck Gant < 2018 1st round draft pick
I don't really see gaining a late first round draft pick worth it for trading Manson for Gardiner