Petry is not a rental. He is signed until the end of next year. He is a #2-3 smooth skating/puck moving RHD able to play 20+ mins in all situations.
He'd not go for cheap if Bergevin made him available (which I doubt he will). I think what LA was able to get for Muzzin would be in the ballpark (1st, B prospect, C prospect), but I think there would be many suitors, which could drive the price up. Let's see injury situation/number of teams on the bubble with D needs at the deadline.
From Winnipeg, it would be something like 2020 1st (top 10 protected) + 2nd + 3rd, since I don't see any prospect in their pool that would interest the Habs beside Heinola, Samberg, Niku or Vesalainen, which I doubt are available. Gustafsson does not move the needle for me.