A lot of money seems excessive, at least in Europe. The flexibility market profitability depends a lot on the national market it plays in, as for instance the prices are quite low in Germany/north Europe, but very high in Australia.
Rather than a way of making money, I like to think about flexibility and fast dispatching as an enabler for way more renewables to come online, and that is crucial for the human race right now.
Makes sense -- the grid storage system I read about was indeed the one they built for the Australian wind farm.
I think there's a big gap for both types of storage - fast dispatching for intraday demand variations, replacing gas peakers, and more static storage as in the OP for multi-day gaps in renewable production such as periods of high pressure during winter when wind speeds and irradiance are both very low.
Rather than a way of making money, I like to think about flexibility and fast dispatching as an enabler for way more renewables to come online, and that is crucial for the human race right now.