If a building contains 20 units which rent for $1500 each you then need to adjust for a vacancy factor, let's say 10% so we have an adjusted $27k in income!
Then using the 50/50 rule we will allocate $13,500 dollars to expenses and $13,500 to debt service and cash flow.
Money left over after expenses is held as prudent reserves for repairs, remodeling, appliance replacement, roof replacement, etc.
Value of commercial apartment unit’s is by cap rate! A class A building may be a 4 to 7 cap rate while class B may be an 8 to 11 cap rate and class C may be a 12 to 15 cap rate and class D 16 cap rate and beyond!