Cost of ownership is essentially just the initial cost. However I am sure you will want to customize / tinker with it as many of us do. These are strictly optional costs though.
Since I have had my OneWheel + I have put 1250 miles on it and my only cost has been buying a new tire ($65) and having it installed ($20). It takes an incredible amount of abuse and still keeps running like new! I have launched down a flight of stairs, ridden it through countless rain storms, ridden it on the beach through salt water, and sent it somersaulting across the concrete countless times. And no issues!
In contrast, I have an Evolve Carbon GT that requires constant maintenance and tinkering just to keep it running. The Evolve seems to be in the shop as much as I ride it. Where as the Onewheel is so easy and it just works.
To further expand on the cost of ownership of the Onewheel you can just focus on the expenses, you need to also look at the cost savings. Savings for me include, no longer pay for a parking spot at work $120 a month, don't pay for gas to drive to work everyday, I get to work in half to the time, I now drive my car maybe once a month versus everyday. I was putting about 15k miles on my car a year and this year I have put less than 5k.