They added a sink, gas stove, electrical outlets, and a bedroom. 

Do you trust pex for gas?  

The time and gas this will save you driving to a clients location will easily pay for itself in a couple of months (depending how busy you are).

In SFH Id have them pay all the utilities, in MUs electricity and gas.

Oklahoma City and Tulsa are certainly exposed to the turmoil in the oil and gas industry, but the effect that low oil prices will have on rental rates and acquisition prices remains to be seen.

IS there Natural gas available to the property?  

My experience is heavy in oil, gas, minerals, and more recently, commercial buildings.Have owned 3 rentals in the past and currently have 1 under lease for a five year term:  Class A - appreciation focused, but CFs at ~$2400/yr. 

Monthly Income 9,175Gross Potential Income 110,100Vacancy 5%Effective Gross Income  104,595Taxes 1,799Insurance 3,000Water & Sewer 1,374Electric/Gas 562Management Fee 5,230Staff Payroll, Landscaping 900Maintenance/Repairs 5,230Admin Costs/HOA 1,500Security 261Replacement Reserves 5,230Trash 450Other Expenses / Adjustments 1,496

List of items I know I need to account for: watertrashrecyclingheatinternetelectricVermont Gas rental (depending if the unit it rented, but based off of what I have looked at most are - around $17 per unit a month)yearly taxesrandom repairs/updates/discretionary funds (this # is important for me to know because many of the duplex's in my purchase price range need a decent amount of work done on them)I just have a feeling I am forgetting something and would love to see how others calculate their costs. 

In my 4-plex, I pay $300-$400/month for water/sewer and gas (hot water heater).