Power Orbs and the Event

[Mythic]

The cost goes down at lvl 20 or 21 then gets back up at lvl 23 it's down again and goes back up, and lvl 25 is max trying to get orb 26 it tells you that "you have opened all the available orbs".
Also there are some glitches when you enter/leave party, go into challenge, the cost change back and forth between 0% Deconcentration and what ever level you have it at. Just give it some time or refresh and it should sort out.