Young top 6 centers just off their ELC are rarely traded so it's really hard to place an exact value on PLD by comparison. Dubois is currently in his D+4 year. Here are some examples of other top 6 centers traded around that age:
Zibanejad (D+5) - traded for a more established (at the time) center, along with a 2nd.
D. Strome (D+3) - along with another former 1st rounder was traded for a D+4 1st rounder C/W in a "change of scenery" move
O'Reilly (D+6) - traded for 2 younger 1st round prospects and a 2nd round prospects, as well as a 2nd.
J. Staal (D+6) - traded for a good defense prospect, a D+5 promising young player, and a 1st.
Seguin (D+3) - along with a decent defense prospect and a cap dump, was traded for an established top end winger, a promising D+4 winger, and a solid defense prospect
These are the ones I can think of right now but anyone can think of some more examples that would be great