This plan is great for taking out bandits, bears and trolls. However it's not so great versus undead (while leveling) and dragons (always moving, or in melee range).
I can't really determine how much (base) magicka you'll need because there's a ton of unknowns.
Let me make some presumptions: You wear armor and not a robe in the chest slot. You don't want to use magicka potions. You don't have spell cost reduction or magicka increasing enchantments.
Calm is the bread-and-butter illusion spell while leveling. Considering perk and skill discounts, it will cost 40-60 magicka to cast. If there are 4 baddies, you'll want to cast it at least 3 times, but 4 times is better. If all baddies are calmed, you are out of combat and get out of combat mana regen rates (better than wearing a robe). In general, if you can calm everyone, during that 30 seconds you'll get back enough magicka to do it again (assuming no misses).
Invisibility is the crowning spell of the illusion school at 75 skill. Considering perk and skill discounts, it will cost 120-150 magicka. I don't have practical experience in using it to break combat (hope to soon), but if it does you'll get back that magicka before needing to cast it again.
Harmony is the most expensive spell of the illusion school. It costs ~310 magicka considering perk and skill discounts.