It's really hard to pin down the value of a guy like Buchnevich because of that impending contract situation. He's not in that pure "rental" market where you've got a lot of baseline comparables to work from.
But a lot of it also kinda comes down to what sort of contract you think Buchnevich is going to want as a UFA after next year. You can't spend multiple premium top tier futures assets on a guy you're not sure you can afford to keep beyond that. Optically, it's suicide for a GM to do that kind of thing.
So it puts the acquiring team in an awkward situation, adding leverage for Buch's camp because they know the GM can't afford to walk away.
You're realistically looking at $9M+ at that point, on more or less a "retirement term". Is that something you're comfortable with? The answer to that question ultimately has to factor into the equation of what you can afford to give up in terms of "assets". Especially when it's compounded by the risk of the player deciding to just walk for a huge payday somewhere else anyway.
But...he'd be a game changing acquisition for a playoff team looking to add something big to really put them over the top. Few other wingers who are even potentially available would have the sort of impact you get from a big, robust, ~PPG two-way winger with a bit of center versatility...right in his prime. And his contract for this year and next is very reasonable and at least within striking distance of fitting into a lot of team's limited cap space, with a little bit of finagling.