The key thing is how Montreal has a very young defense, so keeping a veteran around makes sense; his leadership and experience is valuable as the team has young RD like Reinbacher and Mailloux likely joining the team in the next 12 months.
This is the opposite of the Edmundson situation last season, where Montreal needed to make room for young LD who'd already shown they were ok in the big leagues, and they still got a 3d and a 5th, despite serious ongoing injury concerns.
So, a price of a 1st, as well as a 3d or so for retention, wouldn't be a bad guesstimate of what he does for, if someone steps up (and I'd really rather they don't, and we just get a 2nd in a year for him, as a pure rental)