The only other things I would remember to factor in is the initial cost of furnishing the unit (obviously completely filling a unit with everything from a couch to TV to plates and cups and linens isn't cheap at the get-go.) and then the costs of expendables (toilet paper, paper towels, dishwashing detergent, cleaning supplies, etc.)

I poured a few bottles of Dawn dish detergent over the next 12 months and watered it in until grass finally began to grow back.I did all the work myself (except the new 200 Amp service) over a fourteen month period, including painting inside and outside.

@Matt ClarkI replace my coffee, creamer, and laundry detergent the most because I offer it for free.

This is the one i bought PowerBoss Gas Pressure Washer 3300 PSI 2.5 GPM Powered by HONDA GX200 with 30’ EASYFlex High-Pressure Hose, 5 Nozzles & Detergent Injection

If you go for new ones educate your tenants to use less detergent or you will end up with useless service calls-don't ask how I know!

We’re quickly realizing that our trips to Walmart for consumable supplies (toilet paper, paper towels, dish detergent, etc) are becoming far too frequent.For the people that have multiple units, what is your strategy for buying and storing these supplies?  

Skimp on light bulbs, laundry detergent and paper towels. 

Duct cleaning costs $280-400 depends on detergents and kind of service, numbers of openings - big house might get more expensive. 

Dumping coffee grounds or grease down the drain, using powdered detergents (both laundry and dish) and flushing feminine hygiene products are killer on your septic system.  

We eliminated them in our office with a nightly regiment of Dawn detergent down the drain for a couple of nights.