Owning a property is work - sudden repairs at an otherwise busy time in your life. Phone calls and customer service for tenants. New tenants. Paperwork. It's partly the time actually worked, but also the unpredictability and out-of-hours nature of the work.

Looking at the model I guess the nearest factor is Annual Costs (maintenance). But it doesn't feel properly measured.

I note that for someone on Hacker News, it needs charging at the day rate for a programmer. Or even as an opportunity cost - charging with a risk-based possibility of the reward of starting a successful technology startup. Or of spending more time with family or in sunlight, amongst trees. At peace.