The thing is, you don’t know it.
You can fly a shiny covetor for months without being ganked. And then people notice you and gank you every day.
Or you can fly in a procurer and get ganked from the start because they don’t like competition.
I mean, I agree that theorically the total isk/h should be average_yield * average_price - costship * gank_frequency, but the gank_frequency is not something that is fixed.
If you mine in a conduit after a friend cleaned it in a 1.0 system, it’s no the same as mining in an ice belt in a .5 system.