The comparable that hockey pundits have used is the Jake Muzzin trade from last year which was a 1st round pick, Sean Durzi (2nd round junior prospect), and Carl Grundstrom (2nd round AHL prospect.) Basically a 1st and two 2nds for an ~40 point, #3 defenceman with another year on his deal. That's the base line value for Petry. If Bergevin is offered less than that then there's no point in trading him this year.
Winnipeg is almost assuredly on Petry's modified NTC list, but I could see them paying that price (if not a bit more) for Petry at the deadline.