If you try gauge the price from that cavern storage (estimated 200 million euros) then lets say it has some economies of scale, maybe this sand battery costs like 500k-1M euros.
If you just heat the storage on cheap hours and output at high prices. You could in theory cycle (24*365/2) hours at 1 MW so 4,4 GWh of energy per year. So how could be the project profitable?
In 10 year period with full utilization price differential would need to be 22.8 €/MWh to break even with 1M investment, then add operating costs, loan interest, etc. With more realistic utilization it would need to even higher.
Maybe you amortize costs over 20, 30 years. But its's even more uncertain what cost of electricity will be. I think it only makes sense if this storage allows them to electrify heat production replacing much more expensive current production.