Ottawa certainly DID give up quite a bit to acquire Duchene, but as was mentioned several times already, they HAD the prospects/picks to give while still maintaining a pretty good pool of talent below the NHL level.
Also, if Duchene helps Ottawa to a championship (even if he leaves as a UFA in two seasons) it would all be worth it for the Sens.
Granted, the Sens winning a Stanley Cup in the next two years will NOT be an easy thing for them to accomplish, but it is very possible nonetheless.
Also, while only the two parties involved know for sure (the Turris camp and Sens management), I suspect there was more going on behind the scenes that the public simply isn't aware of.
Some type of breakdown in the relationship between Turris and Sens management.
It has been said elsewhere that Turris and his agent can be difficult to deal with.
Maybe so, maybe not...but IF so, perhaps Ottawa saw the opportunity to upgrade (however mildy) with Duchene, and even though it cost them what it did (again, remembering they had the pieces to give), made the deal to NOT have to deal with Turris or his agent any longer.
All speculation of course.
But for a team like the Sens who have core players who are either in their prime NOW or are exiting their prime, they'd like to win as soon as possible and they obviously felt the price for Duchene was an acceptable consequence for a chance for the former Colorado player to help them achieve a championship now.
And who knows...even when Duchene's contract is up, he may like playing in Ottawa enough to sign an extension that would fit what the Sens want to build around the rest of the team with.