You can look at the assumptions under https://model.energy/ which were based on cost data in Europe (I believe there's a link to the source).
Per-kW cost of electrolysers is already 1/2 of the total per-kW cost given there in the 2030 assumptions (but that may include other equipment).
Cost of storage caverns is well known from natural gas, as little as $1/kWh of capacity. Cost of combined cycle plants to convert the hydrogen back to power is also well known, as these will be nearly identical to natural gas fired CC plants (just the details of the combustors will change.)
Per-kW cost of electrolysers is already 1/2 of the total per-kW cost given there in the 2030 assumptions (but that may include other equipment).
Cost of storage caverns is well known from natural gas, as little as $1/kWh of capacity. Cost of combined cycle plants to convert the hydrogen back to power is also well known, as these will be nearly identical to natural gas fired CC plants (just the details of the combustors will change.)