Discount Factor for Vacancies

[Chris Carroll]

Do any of you use a ball-park factor when buying properties with vacancies? You'd be assuming the risk that the unit(s) will remain vacant for 6-12 months.

Example:

You're looking at an office building with 2 out of 4 units leased (say, for the next 5 years).

The rent from all 4 would produce 100,000 NOI.

Property is worth $1,000,000 fully rented.

All things being equal, what would you knock off your asking price? 

Sorry for asking the question like a standardized test but couldn't think of a better way to express it.