Since there is only 2 years left on his contract before he becomes a UFA, the return would be probably subject to Karlsson signing an extension with the team. Since he does have a modified NTC, it is probable that it would happen. However, the sticking point will be how much that extension would be, and for how long, since he'll want top dollar.
If Karlsson agrees to sign an extension at a reasonable amount, then the return could be quite significant. I wouldn't be surprised with a 1st, a player and 2 prospects, and a conditional 1st.
If the Sens were to try to trade Karlsson without an extension, I think will only get a 1st, a player and a prospect. The reason why it's so low is because of the difficulty it could be in signing Karlsson, and the potential for Karlsson to want to test free agency.
Melnyk/Dorion wanted Duchene bad, and gave up a lot for him.
I can also see the Sens settling for less if they don't believe Karlsson is in the long term plans for the team. Much like how they gave up Turris for very little, since he wasn't going to be re-signed, and they needed the cap space.
With Stone and Ceci to be re-signed, and Phaneuf and Ryan still on the books, cap space could be an issue, especially if they want to sign Duchene to an extension.
But if you are going to trade Karlsson, then it's time to blow up the team and do a full re-build, which I don't see happening. You don't have anyone else to build the team around, and you just traded away a ton of assets to get Duchene. They are going to stick it out with the path they are on, and tweak it however they can.
I think the Sens should focus their energy in moving Phaneuf and Ryan's contract, just to free up some cap space. But good luck.