Kostin makes sense for multiple reasons. We probably have to lose Evander Kane after this season because we need cap space, having his replacement ready to go with Kostin, who is younger, would be good forward planning as well, but if you wanted to really bully an opponent in the playoffs, having all three of Podz, Kostin, and Kane in on top of having Nurse on the back end could in a 7 game series intimidate an opponent and wear them down for sure.
It doesn't have to cut into the D budget, if he's effectively coming in retained at 1 mill it just means you demote someone off the forward roster to bring him on.
If he plays well here (for whatever reason I think he just likes the atmosphere/media of the Edmonton market, while some players wilt he seems to enjoy it), then basically you've acquired a 1st round pick back, you can at least look at it that way, since we've done f*** all (especially at forward) to have success in drafting it would be nice if Kostin-Podz developed here with Holloway, McLeod, etc. gone and guys like Yamamoto busting.
The tricky part will be just paying a 3rd team to retain on him, that should be easier done the deeper you get into the season, since SJ cannot retain.