Separate Maid Unit in my Coop - how to price it?

A separate Maid unit in ground floor of my coop in Manhattan is now available. I can’t find any historical price records for these separate units in my building, and i cant find any knowledge of how to assess the real estate value of such a unit. The unit can only be sold to owners of building, so obviously a limited amount of buyer. and it is a 250sqft in original condition on ground floor with no kitchen - but with full bath - next to service elevator. anyone can help with some knowledge on how to value this? is it a discount to average prc/sqft in building. should I comp it to a studio in the neighborhood and apply a discount? please share your experience and knowledge. thanks