Any leasing contract is likely to have extension clauses to protect the operator from a massive increase in price without time to find replacement and put into service replacement stock. As such if leased at an uneconomic rate it may be tricky to move them onto a more suitable rate when market conditions are better. The storage and other costs would be small compared to the potential lease amount.I'm assuming there is a transport cost to storage, storage space cost, and still some basic maintenance costs that will be accrued to prevent them becoming unusable again. The economic incentive is to get them used, even if the lease is low it is still saving those other costs (or moving them to someone else). So, I'd suggest it's either they get leased out on the cheap, or end up scrapped fairly quicky. If the initial 10 year lease was high as suggested they might have covered their cost already (or be well on their way to doing so), so a low lease might be less of a problem.
Could another potential use be to replace some of the aging EMU stock around Glasgow?
If scrapped any shortfall from the book value on the accounts to scrap value would have to be recognised making a dent in the accounts. At that point they might be prepared to accept a low lease rate if the financial case is better than scrapping. It sounds like they are going to warm store so their strategy might be hoping that passenger numbers rebound quicker than the government expect with few quick options to expand capacity meaning they can charge a premium price for the 379s.