It sounds like you are leaning twards dividing common property up and adding it to the individual lots. If this is the case, I would warn against it as it is legally complicated and there are easier ways of doing this.
We had a similar issue in our building and we erected storage cages then rented them out to the owners on a first come, first served basis (there weren’t enough spaces for everyone). In your case the owners collectively own the space anyway but the value is what they would be prepared to pay for the cage rather than the space).
I would find out how much it would cost to create individual storage cages for each apartment and then rent them out to owners to cover the cost of the caging. Allocate cages to owners by ballot and if they don’t want to pay, then offer them to those who want extra storage.
Once a year (at the AGM, for instance), you can ask the owners who have passed on the storage if they want to take it up. Otherwise it gets let out to other owners on a first in best dressed basis.
I think the OC needs to keep control of these storage spaces even if owners are guaranteed use of the cages, one way or another. The rent from them can go back into funds to help with maintenance and management.
As far as an absolute value goes, call around and ask what commercial storage firms charge per cubic meter in your area and then knock a bit off that. But there’s no absolute figure because storage space is only valuable to those who need it. It really comes down to what your residents are prepared to pay and how much more attractive the storage makes your apartments to potential buyers and tenants.