Do you mean thermal efficiency? I pegged it at somewhat less than 50%, like most power plants:
You have: 1g / (7 g/mol / avogadro ) * 17.5 MeV
You want: kWh
Since a typical household electricity use is about 9000 kWh/year, at 35% efficiency, that would be several years worth.
If you mean transmutation efficiency, it doesn't matter. You keep recycling the lithium until it converts. As I pointed out, the overall cost of that process could be high, but the cost of the raw lithium is insignificant.