I've done the 1st three units and the cost was really high between the costs of the units themselves, the plumbing electrical, HVAC/venting, and the closet build-out to house them.

The electrical was upgraded in recent years as well.

The Air conditioning wasn't turned on before she came in and it was too hot (tried saving electricity no one in home). 

Now it seems I have the bug.

Also take a look at utility costs - if you can separate them out and have tenants pay their own, it's helpful, because they treat the heat and electric use very differently if they are on the hook.  

I think the last one we did on a new build (2012) was >$1500 with the tank plus the ancillary costs of plumbing & electrical hookups. 

Electrical and plumbing was professionally inspected and repaired where needed. 

I start with demo, foundation and plumbing, then roof and electrical work if needed, paint, kitchen, bathrooms, floors, baseboards, light fixtures, etc.

I’ve not heard of using a hot tub to cool off.Yes there is a cost to maintenance and electricity, however in my mountain market it’s probably worth $10k in extra bookings/year.

Gas, electric, water, sewer and trash.Get Ready for Rent:- Change the locks.