Quoting: Mike
What's the value on the young player who's 6'5 230lbs when healthy has 40+ goal per yr in him power forward ..
Comparables are: Evander Kane, Anders Lee, JVR. JVR was traded for Luke Schenn. Kane went with Bogosian as a package deal for Myers, a 1st, and some middling NHLers. Lee was never traded, but did record his best seasons at ages 26 & 27. The others hit 30 goals before Mantha's current age.
As Newhook was just awarded Hockey East Rookie of the Year AND ranked #7 on ESPN's NHL prospect rankings, I'd consider him a top prospect. For reference, Byram is #5.
Zadorov has proven he can play top-4 minutes on a playoff team. He's a top-4 defender on just about every team except the Avs.
One thing to note when looking at power forwards: they have more <50pt seasons versus >50pt seasons. With Mantha, he'd be entering his prime production for the next few years. But, given that you aren't getting more than a 4-5 year player out of Mantha, I think a solid NHL defenseman and top prospect is the appropriate return.