Hey, I've had fun writing it and yes, you're right about the cost of mining rigs. I think I've also mentioned it in the article along with the costs involved by maintenance, hiring personnel, and taxes/rent.
I like to believe that this is an informative piece for the community. There's so much negative news going on and today I've logged into Facebook to see all the "I told you so" wiseasses who proclaim their intellectual victory as predictors of the decline. Yes we were in a bubble, but any price over $6000 is realistic at this point. And let's not forget that the rumor in November 2017 was that the price is only $1000, which means that the difficulty of solving problems has increased so much that $50.000 by the end of the year doesn't sound that ridiculous.
Now it's all just speculation that the price of bitcoin now looks so exciting, like a roller coaster.