I don't see why Weegar's trade value would be all that different from Karlsson/Ekholm/Lindholm/Muzzin/etc.
I'll take a crack at it
- Karlsson, despite coming off of a ridiculous season is/was risky and 33. He's a boom/bust player and it was a boom/bust team looking to roll the dice in a boom/bust transaction because it's either win now or die. There weren't (m)any suitors, so you're kind of bidding against yourself.
- Ekholm as mentioned is 3.5 years older than Weegar. 1st + 4th + 3.5 years of Weegar = ????
- Lindholm was expiring (though signing an extension the day after means Boston was obviously given permission to talk to him, but still this limits the trade field to teams Lindholm was willing to sign long term with) and still netted a 1st, 2 2nds, recent 1st rounder Vaakanainen. Lindholm was also kind of considered to be a diminished version of his previous self at the time of the trade. His reputation repaired itself last year in Boston.
- Muzz was signed for 1 more year, not under control for the rest of his prime and still cost a 1st, and 2 recents 2nd round pick prospects.
A lot of this comes down to the circumstances under which the players came available. SJ finally gave up trying to get max value for Karlsson after a solid year of trying to rape the world. Nashville finally traded Ekholm to tank when he was 33. Anaheim didn't give up Lindholm until the last minute before they would lose him for nothing in free agency. LAK was mid tank and knew the time to move Muzzin for as much as possible was then.
I don't see Calgary being remotely as convinced that now is the time to move Weegar without an overpay, just like trying to trade for any of the above before their clubs were ready to move them would have required an overpay. Calgary has lots of stuff they could move to clear the decks now and aim for a quick retool (as a rebuild is unlikely to be approved by an ownership group currently building a 1.2 Billion dollar arena)