going from 6 to 4 is significant. If it only costs a pick 60-70 then we would have seen such moves more often.
Its costing you alot more
It depends on the draft year. This year is interesting, the cluster of D reminds me a bit of 2012. Columbus traded #4 to Carolina for #8 and #59 in 2004 when there was a perceived drop after #3. Unless a team is 100% certain they'd still be landing their guy at the later spot, they'd simply want a bigger price tag to risk it. This crop is interesting, namely this grouping of D which reminds me a bit of 2012.
Sounded like something was close in 2016 between Columbus-Edmonton-Calgary swapping picks 3-4-6 but Edmonton (?) got cold feet. Columbus would have moved from #3 to #4 to take Pierre-Luc Dubois; They'd take him at #3 but in the proposed trade they would have added some additional asset. Not 100% sure who Calgary was targeting at #3, but I believe Edmonton was aiming for Mikhail Sergachev at #6.
In other years, it comes down to the players. Boston offered #5 and #37 to Washington for #4 in 2006, but that didn't go through since the Bruins intended to take Nicklas Backstrom. Once they told that to the Caps, then talks fizzled out pretty quickly.
And then there were some of the goofy trades from the late 90's / early 00's. Brian Burke moved two 3rds to move up from #4 to #1 in 1999, then traded back a spot to get a 3rd. In 2002, Florida gave Atlanta a 3rd rounder to promise not to select Jay Bouwmeester at #2. At the same draft, Anaheim gave Nashville a late 3rd rounder to not pick Joffrey Lupul at #6.