Buying common space/hallway space in co-op

Coop board has more or less full discretion. Typically latest comparable sale price per sq ft less some cost of finishing would be fair. They may decide to cut additional shares and your maintenance will go up proportionately.

If I were on my co-op board (which I'm not, because I'm a broker) I would do it the other way... figure out how many shares attach to the space and then sell it to you at the prevailing cost per share. So if 7th floor shares tend to be valued by the market at 10% more than 4th floor shares, because of the floor differential, but you're buying 7th floor space, then you pay 7th floor prices for that space. But 300 is correct that the board has a lot of latitude to work through this.
ali r.
{upstairs realty}

When hallway space purchases have been requested in my co-op we generally have had an appraisal done of the space in question to determine a fair price (shareholder buyer pays for the appraisal). Monthly maintenance is determined in a similar fashion to what Ali describes above.

Appraisal. The process said something about the space if sold being useful only to one party (the owner of the 2 apts adjoining), so the value was slightly less than the prevailing price per sq ft from other apartment transactions.

There was a deal done at 2 Horatio St a couple of months ago.
https://a836-acris.nyc.gov/DS/DocumentSearch/DocumentDetail?doc_id=2021072601235001

@30--that seems high--wonder how large the hallway is?

Boards usually have a policy in place
Pricing is valued at a discount to going $/sf

An example could be 50% valuation of the last 5 sales in the building's $/SF:

$1000/SF ( last 5 sales) x 50% = $500/sf