> Since the reward has now halved, I think the electricity spend should also drop by half

I doubt that. Look at the recent oil production issues: despite demand falling off a cliff, and the price of oil following it, production (supply) hasn't dropped anywhere near enough to match the fall in demand. It's basically a game of chicken--keep going at full force, eating your losses, in the hopes that your competitors are weaker and will be forced to fold before you will.