If this is an complete alternative to quest markers I think it will be an improvement.
If it's an enhancement to Quest markers or necessary to use because of a lack of directions/ roads / signposts / explanation. Then ill be a little disappointed.
Since this is apparently a navigation "feature" an not really a "spell" (you cast it like a spell, but I think that's just convenient developing / immersion rather than adding an on/off tick in the options) it will not bother me if there is no mana magicka consumption. After all, the magic compass in oblivion had no mana Magicka consumption, and there was NO outcry about that. Why should this be very different.
Because:1) it's under the list of spells
2) Todd specifically calls it a spell
3) it's something you equip in your hand like a spell
4) it's something that you cast like any other spell
5) it's something that looks like an actual spell.
Enough reasons for you? It should drain magicka, like any other spell. It would make things more tactical and make much more sense, as stated above.