Really tough to gauge what it would cost for Buchnevich. Might be less than what people expect given the low prices this year. How much more than a 2024 1st and Berggren? You could add a conditional 2025 Draft pick (not a 1st) if Buchnevich re-signs and/or one of the defense B prospects.
The thing with Buchnevich is that his current Cap hit is low for what he brings. He'll be 30 when his current deal expires and rarely plays a full season, so he's not going to get a long-term deal from anybody. If he fits in Detroit and likes it he might be amenable to a fair 4-5 year deal. And if extension talks go nowhere, and he's having a decent season, you'll be able to get almost as much for him at the TDL as it cost to acquire him in the first place.
The RD piece we need is not likely to be found IMO until summer. SY has been reluctant to give out long-term deals, I doubt he'd be happy with Parayko still having six years left, the acquisition cost adding to that. Other guys will be available for "free" this summer, or like a Larsson be cheaper to acquire.