This historic high of already mined, refined, processed 99% pure lithium metal delivered in China is $63,000usd per metric ton. ($28/lbs)
The 24kWh Nissan leaf battery uses ~9lbs of lithium. The active mass of lithium that actually moves from anode/cathode when you charge/discharge the battery is ~4grams.
That's a cost of ~$257 in lithium (which nearly all becomes part of a complex cathode structure), and of that cost, the 4g of lithium ions that are what give and accept electrons to store and discharge the energy the packs holds are ~$0.51.
If the Nissan Leaf battery costs say, $14,000usd to make, the worst case lithium cost in the battery is ~1.8% of the price of the battery. Of all the places to focus on cutting costs in the battery, even if you take that 1.8% cost down to zero, you still didn't make any noticeable impact on the cost of the battery.
The awesome part of Mg ions is the ability to be ++ charge carriers rather than lithium's ability to only be a + charge carrier. That means each Mg ion that passes through the seperator and swaps sides gives you 2 electrons rather than one, and the voltage of those electrons it gives is high. If you could wave a magic wand and replace those 4g of active lithium ions with the same number of Mg ions (which would weigh ~13g rather than 4g) in a battery (and somehow have the solvent work, and seperator work, and annode and cathode work etc etc), you would get an immediate boost of about 250% more energy stored in that battery. Since Mg doesn't tend to form dendrites like lithium (the reason the early metalic lithium anode batteries burned/failed, and why you shouldn't try to recharge lithium primary cells etc), Mg can avoid needing a bulky/heavy medium on the anode to store itself, it can just plate sheets of Mg metal (no lithium in metalic form should ever exist in a healthy lithium ion battery). This saves the volume/weight/space of needing the graphite layer bonded to the copper foil we all currently use in our various types of lithium batteries, and as a result enables the energy density to climb 30-40% ontop of the ~2.5x gains from 2 electrons and higher voltage, for a total of about 3.25x higher energy density than lithium ion batteries.
That's the part I get jazzed about over Mg based batteries rather than the cost of Mg being cheap and abundant. I want a 3kWh battery on my bicycle taking up the space and weight of a little 1kWh battery. Or a Nissan leaf or Zero/Brammo/lightning/mission motorcycle that does ~350miles of range on a charge rather than ~100miles. That would be super rad for the electric revolution replacing fossil fuels.
