The price of any commodity tends to gravitate toward the production cost.  If the price is below cost, then production slows down.  If the price is above cost, profit can be made by generating and selling more.  At the same time, the increased production would increase the difficulty, pushing the cost of generating towards the price.

In later years, when new coin generation is a small percentage of the existing supply, market price will dictate the cost of production more than the other way around.

At the moment, generation effort is rapidly increasing, suggesting people are estimating the present value to be higher than the current cost of production.