I don't think this question can or will be answered until the NDA is lifted. I will unbury it when the time comes.

Question: Anyone able to confirm if magicka costs will be based on your max magicka (Speculation: Siphon costs 10% of max magicka so if you had 200 magicka the cost would be 20.) or if it's just a flat cost based on character level/spell level?

I think I have seen an answer to that but it would probably fall under the NDA.

If you are for example at level 10 and your spell costs 50, it will still cost 50 when you are at level 15 and so on.

When I was scaled up to 50 my spells ate my magicka MUCH faster, they don't consume a percentage, but instead the costs scale with level!

spells become better when you level and cost more... it scales up

I didn't actually check this while playing, but the costs do go up. I don't know whether it's entirely based on character level or also factors in the skill level/morphs of the skill in question, though.

Since you gain health/stamina/magicka as you level, even without spending your level-up boost, it shouldn't be too difficult to keep up, and you can use other attacks or non-magicka abilities in the meantime. (Magicka also regenerates at a reasonable rate, and you can boost it further with talents/gear/etc.)

Some morphs did not increase the mana cost while others did. There are also reduce spell cost enchants and passives that a player can get.