Over at BraveNewClimate, there are some plans and cost estimates for a large (9 gigawatt) pumped hydroelectric storage facility in Australia. Two reservoirs separated by 875m of elevation would be joined with a 53km pipe. The total estimated cost is $6.6 billion, or about $744 per kilowatt of storage. That is a pretty expensive proposition, given energy prices today. That being said, such storage facilities are likely to be essential for increasing the share of total energy usage that comes from renewables, across the medium-to-long term. Right now, most Australian electricity is generated by burning coal.
I wrote previously about a similar facility in Wales.