Again, the issue here isn't the individual value of one of the Flyer assets. Montreal, with Hart and Provorov added, is probably trading the 10-15 pick in next year's draft in that proposed trade. With no other assets. And two cap dumps. Can you find the last time a team offered a 1st pair defenseman and 25-yo #1 goalie for 2 cap dumps and a mid-1st a year away?
The comparable for Hart on the high end is the Cory Schneider trade. Schneider had less of a track record than Hart, but Hart's numbers have been lesser than Schneider's (granted that Schneider had a much, much better team in front of him), so we're looking at lesser value than the 9th pick in the draft. I'd say Hart should return something between the 12-18th pick. Montreal isn't likely to be drafting exactly in that range for the next little while without some significant improvements to their roster (frankly, neither will the Flyers) so we'd be looking at Hart + assets for Montreal's pick, or prospects the Flyers value at about that same level. I don't know Montreal's prospect pool all that well, so I won't start throwing names around, but that's about where I see Hart's value, and why.