In particular, hydro storage can be built in hundreds of times as many places as watershed hydro generation. And, there are numerous other practical storage methods.
Ready combined-cycle gas generation capacity is more important, most places. Those will be incrementally converted to consume imported synthetic ammonia.
Synthetic ammonia that comes from where? Ammonia is currently produced by steam reformation, which emits CO2. You're talking about electrolyzing water to produce hydrogen, and using renewable energy to carry out the Haber process. Nobody is using this strategy of energy storage, so it's priceless in a very literal sense: there is no way to estimate how much such a system would cost. I'd take an expensive solution over a priceless solution any day.
Insisting technologies that people are already spending billions of dollars on building out were not first shown to be viable is a peculiar position to take.
Even so, do you have another figure? This is a complex molecule, it is naturally inefficient to produce compared to hydrogen, which is below 50%.