A real estate appraiser can probably help you with this. They should be able to do a market analysis and compare rents of other similar apartments/buildings in the area. You will have to call the appraiser as an expert witness to admit the report into evidence unless he agrees. You may want to consider hiring an attorney to do the hearing
Any real estate agent in the area can give you an idea of what the fair market rent would be. You can also try to see what apartments in the building rent for.
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This advice is not meant to create an attorney-client relationship and is a general anwer to the question posed.