I don't think Harper's current struggles would affect his price. He's still Bryce Harper and there's too much other baggage (sentimentality, the Nats not being totally out of it, even the QO comp pick) for the Nats not to insist on a premium price. Particularly from the Phillies. And, as with Machado, the Phillies probably aren't as motivated as other division leaders with more advanced win-now windows.
Plus, what if he doesn't bounce back this season (and why would he, at this point)? What would you actually want to give up if he only adds one win compared to Williams?