Assuming it's a capital repayment mortgage, you're neglecting that you are erasing debt, only the interest payment is really a cost here.
But yeah I think it makes sense to exclude it anyway, needs/wants might change and you no longer have the room(s) available to let or whatever, and it wouldn't really be a useful comparison of your 'side project' earnings to compare a year as a landlord with one not, it's a different thing.
Insurance ($97/mo) + Mortgage ($3,400/mo) + Property taxes ($444/mo) + trash, water ($100/mo) = $4k/mo. My mortgage is new and is only paying down principle ~$120/mo. This means I pay about $900/mo for my room, except I have had an average of $1k/mo in home repairs since I bought the house.
The real value I am capturing is my personal housing _might_ be cheaper than if I rented myself.
I am sure in 10 years, I will be in a good spot, but not today.
But yeah I think it makes sense to exclude it anyway, needs/wants might change and you no longer have the room(s) available to let or whatever, and it wouldn't really be a useful comparison of your 'side project' earnings to compare a year as a landlord with one not, it's a different thing.