I don't think he has negative value but I'd be willing to be convinced that he does.
I don't think he has particularly high positive value (and never did,) but his contract is more or less market rate. It's what I'd expect him to get as a UFA, more or less.
I made this argument on Twitter but people let the quality of the team impact how they view contract value, and it really shouldn't. I haven't heard anyone argue that Mika Zibanejad, for example, is "negative value." If the Canucks were actually a good team, nobody would say it about Miller either.
Negative value means that a contract is overpaid to the extent that a team who wishes to trade it would have to pay to get rid of it rather than actually get something in return. I am not convinced that Miller's contract is there, nor Mika's.
Now, do I think Miller would return particularly much in a trade? Not necessarily, but "low value" is not the same thing as "negative value."
I think, cap space issues aside, teams would pay something to acquire Miller's contract. That makes it positive value, albeit potentially rather low value.
Again, I'd be willing to be convinced but I haven't really seen the evidence.