I'm not here to say this deal isn't market value, it very well might be (I don't really know SJ prospects). But I'm 99% sure that MTL will not trade Armia right now for market value.

Habs don't have star forwards (yet...fingers crossed), so they need to capitalize on their depth, to which they have an insane amount of after this off-season. Having someone like Armia, who was on pace for 20 goals, is 6'4'', and skates well on their 4th line (maybe 3rd line if Toffoli plays LW) is what MTL needs this season.
I'm not opposed to moving him, it's pretty much written in the sky that he'll be a cap casualty next year, but unless Habs get an offer they can't refuse for him, they most likely wait till the trade deadline and see if they are in the playoffs (and just keep him as their own rental), or out of the playoffs and go for market value then.

That's fair. I think Armia is such a good fit for that 3-RW spot for SJ that they should be willing to pay slightly over market value to get him. Like a Gambrell and a 2nd rounder for example. I doubt that's what it would have to take but if it would, it'd be a price worth paying, especially if they get a good sense they can sign him to an extension.