Normally flying up or down does not give or cost extra movement, as covered here.
However, Etherealness spell (duration 8 hours) states the following:
You remain in the Border Ethereal for the duration or until you use your action to dismiss the spell. During this time, you can move in any direction. If you move up or down, every foot of movement costs an extra foot.
For a flying creature, which enters Ethereal plane using this spell, does flying up or down cost extra movement? This is assuming it's possible to fly in the Ethereal plane at all, but I didn't find anything saying it's not.