I don't think Pettersson gets moved, especially at the deadline. That seems like an offseason one to me.
Horvat obviously goes because of his pending free agency and the teams acknowledgement that they can't sign him. The acquisition cost probably depnds on if the Canucks allow teams to talk extension with him or if they sell him as a pure rental. I don't think the price will be crazy exorbanent. The closest comp I can come up with was when the Sens traded Matt Duchene. Duchene was a 28-year old top-6 center, pending free agent. The traded netted Ottawa a 1st round pick, 2 mid-level prospects and an conditional 1st if Duchene signed with Columbus.
I think ultimately if the Bruins are involved the package would be something similar to the Lindholm deal. 1st, 2nd, 2nd, roster player (Vaak), cap dump (Moore). The Bruins obviously don't have those 2 picks anymore so probably have to swap in prospects or ruther out picks.