get the updated rent roll, what is every unit renting for? Estimate the rental rate for the owners apartment.. example - if he lives in a 2 bedroom and another 2 bed in the same building rents for 750, use that to estimate a full rent roll. then add it all up and multiply by 12 to get the Effective Gross Income.
be sure to add in any additional income like a coin laundry etc.
this gives you our Gross Potential Income - then start deducting expenses -
(10% Vacancy) (this accounts for tenant turnover or just general vacancy, raise the % depending on the local market)
(Taxes)
(Insurance)
(lawn maintenance / pest control services)
(Repairs and Maintenance)
(Management Fee)
(Common Utilities paid by landlord)
(legal fees)
(Marketing)
etc etc. make sure the expenses are atleast 50% of the Gross Income.
this gives you the NOI.
then you can determine the price based on the market. Where I live, apartments are trading for anywhere from an 8% - 10% return so i would take the NOI / .08 or NOI / .10 to determine a price.
List it and let the market determine the rest, best of luck!