For reference, coire glas pumped storage in the UK is $1.3 billion and 1.5 GW / 30 GWh. More expensive than China, but not wildly so.
5 days' worth of nuclear power plant's output's worth of storage is 384 GWh. Assuming coire glas prices, $1.3 billion*384/30 maxes out at $16.6 billion or roughly half a hinkley.
That also gives max output of 19.5GW that is fully dispatchable as compared to 3.2GW which comes out whether you need it or not (& must still be paired with storage/gas to match demand).
Lowering the power rating will lower the cost of the pumped storage.
Wind energy is currently about 1/3 the price of nuclear, so assuming 5-6 day lulls it still comes out break-even at the very least.
Until you consider insuring against $800 billion fukushima style clean up costs. If we properly accounted for the cost incurred by insuring against this risk we'd never build another nuclear plant ever again.