Minny got a 4th rounder for retaining around 74k in the ROR deal. Think they or any team would ask for way more than a second to retain 4M on EK. A first and a really good prospect would be minimum I would think.
Minny retained $1.875M on O'Reilly and gave up Josh Pillar for the 4th rounder they received. If you go to CapFriendly's homepage and scroll down it has a recent trades sections with all the particulars including retention and overall cap implications for each trade which is pretty handy.
It might not be the place to find breaking trades on trade deadline day but CapFriendly should have all the trade details a day or two afterwards, except maybe some info on conditional draft picks if that info hasn't been disclosed yet.
Anyways, with regards to retention it seems like it's somewhat variable depending on the trade and teams involved. The difference with Karlsson obviously is that he's got 4 more full years on the deal. If the Oilers could get another team to retain 13% on Karlsson that's $1.5M per year. Is that worth a 3rd rounder or more per year? Probably depends on the team and they'd probably try to extract a 1st + for that much retention instead of a 3rd rounder for each year of the deal. Chicago seems like the ideal team to involve for 3rd party retention in such a situation. Seems unlikely though given the length of Karlsson's deal, but who knows.