Powered Cube's price is crazy high for a phantom cube draft. Literally you get better rewards IRL at most events that have powered cubes than on Arena. Also, it was previously priced lower, so it's not like you can't do it; frankly, it's just not enjoyable to play Powered Cube when the stakes are so much higher, and it stifles creativity—every pick you make when the draft price is that high, you're forced to think about the economics of the game, instead of the gameplay. Each potential "do I pick the more enjoyable card?" is balanced against losing 1500 gems for it, and I think this also encourages more bland deckbuilding as a result. It just doesn't feel in the spirit of the game, it feels like greed.

I can't speak for other players, but for me, if it was 600 gems to enter, I'd be doing tons more cube drafts, and likely losing more gems, because I'd feel freer to build the deck I want. This would still be a net gain for you, and drive engagement rates up.

As it stands, it doesn't feel in the spirit of the game, particularly when the comparison points are non-phantom premier drafts. I can't think of a single IRL event where I've paid more to draft a powered cube than I have to draft a normal set, because players and cube convention organizers understand this.