I'd check your napkin ;)

5% of the world electricity usage divided by wholesale electricity cost, divided by world lithium production in grams ->

(5% * 2.5e+16 Wh X $30/1e+6 Wh) / 8.2e+10 g = $0.45 per gram (or 15kWh per gram of lithium)

The takeaway is that energy is currently unreasonably cheap.