I find that number dubious, since the per-kWh cost of flow batteries should be low: just make the tanks larger.
Not unreasonable. The details are always in the research paper after all.
Increasing the size of the tanks to increase the energy storage capacity, without increasing output power would be... useless.
Extreme example: a huge expensive 100 MWh battery that you can only draw a max power of 100 W from = useless.