Let's say price USD 9000 currently. The correct accounting treatment for the number 1965 * ( 9000 - 4363) may not be a pure profit due to the halving. Instead, part of it shall be the reserve for the higher cost afterward.
Very good point here. Its interesting to see that their expenses for production of one Bitcoin is almost exactly half of what the current Bitcoin price is. with the halving just around the corner it makes a lot of sense, but also shows that they are just breaking even at these figures after the halving.
Even assuming the same mining energy power across all eras, the production cost does not double at every halving. Say, the block fee is 1.5, then the era-specific production cost is x ( 12.5 + 1.5 ) / ( 6.25 + 1.5 ) this time, approximately 1.8 and smaller and smaller to 1.0 in the future halvings.
That said, I don't think (USD of 2019) USD 9000 is the average production cost till the end of era of reward 6.25.