I've been reading a bunch of the discussions where the 50% rule has been brought up. I have to say, I had may doubts that it was anywhere close to that high. So, I finally decided to go back and check the history on both my duplexes and compare. Here's the results:
Duplex 1
I've owned for 10 months. It's a 30 year old property that had been neglected. It needed minor cosmetic rehab work when I purchased it, and it was empty. So, after replacing some carpet, painting the whole place, and some other minor improvents, here's where I'm at.
Expenses $9600 (including everything but mortgage)
Income $17300
Expenses=55% of gross rents
Duplex 2
I've owned for 8 years, and it was 3 years old at the time of purchase and needed nothing but a little touch up painting at the time.
Expenses $31200 (including everything but mortgage)
Income $104200
Expenses=30% of gross rents
Total expenses between the two = 34% I do expect this number to go up a as the property that is now 11 years old starts to need more repairs. However, the property that I purchased last year will not need as much work over the next year or two since I have taken care of most of it. So, I would think that 40% is a decent long term average between these two properties.
In any case, I think the amount of your expenses will be dependant on the quality of tenants that your property attracts, the age of your property, and the number of units (the duplexes probably require a little less maintenance than say an 8 unit).
What Mike is saying about 50% costs is probably a good rule of thumb for conservative cashflow estimates. It'll pretty much garauntee that you cashflow if you use that number when you are looking at purchasing a property.