Both are worth 1st+prospect+low level roster player or cap dump if the prospect is blue-chip. Realistically, Dumoulin is 29 and a top-pairing dman signed to a good value contract.
Rust is a top 6 forward with all the traits you want signed to a good value prospect. You can nickle and dime things based on a granular point analysis but it does detour from the fact that guys like Rust are highly sought after by 31 NHL teams, and for good reason.
I would put Rust's value slightly above Zucker's - 1st+Addison+Chucky as a dump. So start with a 1st + grade A prospect. Hell, even Kapanen pulled the 15ov + Hallander and he hasn't had the production Rust has. So again, somewhere in the 1st + A prospect realm.
Dumoulin, the same thing -- 1st+prospect
Dumoulin to the Habs for 1st+Prospect.
I think that would be realistic. 1st as the base then prospects can differ based upon a few different things. No roster player back, I would want a good prospect. If we are getting someone back that may be of value right away, I would be more flexible on the prospect.
I can also see them being a 1:1 swap where a team trades from strength to bolster a weakness. I've long thought that Dumoulin for JT Miller is a good value swap.