Storage units

Is this a coop or a condo? I bought a storage unit a few years after I bought my condo. The storage unit had a separate deed, etc. My buyer did not want it, so I took a guess on the sales price and advertised it on Building Link for my condo. There was a bidding war on it and I wound up getting almost $1,000 per square foot.

can you sell it separately, or does it have to transfer with the apartment? does every apartment have a storage unit, or are there only a few storage units in the building?

Questions you have to answer in order to have a basis for evaluation:
Can it be separated from the apartment (either separately deeded, or has a rental agreement separate from the apartment).
Who controls assignability? (if it's rented from the building, you may only get to continue and renew the lease, not transfer it to a 3rd party).
How big is it, and what is the access (small storage locker or big walk-in room)?
Can it be used for something else (the infamous "home office" space)?
What is the competition: does every unit in the building have one, or are there just a few?
In the worst case the value is 0, or minimal. Slightly better: it's like having an extra, remote, linen closet. In the best of all worlds, it's like having a 2nd bedroom.

Storage space is really valuable and you don't want to price it wrong. You can call a local storage facility and find out how much they charge for.