I've had artifacts take 2+ million to go from 15-16.
I've also noticed that happens more often during a artifact leveling event, so I'm pretty sure the upgrade rates for 4, 8, 12, 16 (the only ones that count for that event) are nurfed pretty hardcore for the event.
I've also had one or two pop 15, 16 right in a row, though I suspect I dozed off between 15 & 16 so it only seemed to go one after the other. Watching 20-30 failures in a row tends to make one drowsy.
Overall the artifact upgrade system is the most horribly implemented part of the game. You spend days or weeks farming an artifact with the right base stat, speed boots, crit rate gloves, etc (I think its gloves, there so rare I can't remember) with 1 or 2 decent secondary stats in the right set then spend 4-6 mil upgrading and the bonus all goto the crap stat making the piece completely useless.
I get it, you took the lazy way when you made the game and went with RNG instead of doing something sensible that makes the game fun instead of encouraging people to quit.
But that doesn't mean you can't TRY to improve the game for the sake of longevity and overall profit instead of continuing to burn the players and the game for the sake of a quick cash grab.