The ceiling is going to be lower than with lithium. Sodium ions themselves weigh about 3 times more than lithium, for the same +1 charge. So it’s not just that sodium is a certain number of years behind lithium. It’s that it’ll likely plateau at a point permanently behind where lithium will likely be.
Price per kw and price per kwh stored. And price per kwh over the expected lifetime of the battery itself (longevity and reliability and safety and disposal will have to be factored into total cost of ownership).